This PR was merged into the 4.3-dev branch.Discussion----------[Messenger] Add a Doctrine transport| Q             | A| ------------- | ---| Branch?       | master| Bug fix?      | no| New feature?  | yes| BC breaks?    | no| Deprecations? | no| Tests pass?   | yes| Fixed tickets || License       | MIT| Doc PR        |symfony/symfony-docs#10616| DoctrineBundle PR |doctrine/DoctrineBundle#868As discussed with@sroze at PHPForum in Paris I've worked on adding a Doctrine transport to the Messenger component.Actually `AMQP` is the only supported transport and it could be a good thing to support multiple transports. Having a Doctrine transport could help users to start using the component IMHO (Almost all projects use a database).# How it worksThe code is splitted betwwen this PR and the one on the DoctrineBundle :doctrine/DoctrineBundle#868## ConfigurationTo configure a Doctrine transport the dsn MUST have the format `doctrine://<entity_manager_name>` where `<entity_manager_name>` is the name of the entity manager (usually `default`)```yml        # config/packages/messenger.yaml        framework:            messenger:                transports:                    my_transport: "doctrine://default?queue=important"```## Table schemaDispatched messages are stored into a database table with the following schema:| Column       | Type     | Options                  | Description                                                       ||--------------|----------|--------------------------|-------------------------------------------------------------------|| id           | bigint   | AUTO_INCREMENT, NOT NULL | Primary key                                                       || body         | text     | NOT NULL                 | Body of the message                                               || headers      | text     | NOT NULL                 | Headers of the message                                            || queue      | varchar(32)     | NOT NULL                 | Headers of the message                                            || created_at   | datetime | NOT NULL                 | When the message was inserted onto the table. (automatically set) || available_at       | datetime   | NOT NULL                 | When the message is available to be handled                      || delivered_at | datetime | NULL                     | When the message was delivered to a worker                        |## Message dispatchingWhen dispatching a message a new row is inserted into the table. See `Symfony\Component\Messenger\Transport\Doctrine::publish`## Message consumingThe message is retrieved by the `Symfony\Component\Messenger\Transport\Doctrine\DoctrineReceiver`. It calls the `Symfony\Component\Messenger\Transport\Doctrine::get` method to get the next message to handle.### Getting the next message* Start a transaction* Lock the table to get the first message to handle (The lock is done with the `SELECT ... FOR UPDATE` query)* Update the message in database to update the delivered_at columns* Commit the transaction### Handling the messageThe retrieved message is then passed to the handler. If the message is correctly handled the receiver call the `Symfony\Component\Messenger\Transport\Doctrine::ack` which delete the message from the table.If an error occured the receiver call the `Symfony\Component\Messenger\Transport\Doctrine::nack` method which update the message to set the delivered_at column to `null`.## Message requeueingIt may happen that a message is stuck in `delivered` state but the handler does not really handle the message (Database connection error, server crash, ...). To requeue messages the `DoctrineReceiver` call the `Symfony\Component\Messenger\Transport\Doctrine::requeueMessages`. This method update all the message with a  `delivered_at` not null since more than the "redeliver timeout" (default to 3600 seconds)# TODO- [x] Add tests- [x] Create DOC PR- [x] PR on doctrine-bundle for transport factory- [x] Add a `available_at` column- [x] Add a `queue` column- [x] Implement the retry functionnality : See#30557- [x] Rebase after#29476Commits-------88d008c [Messenger] Add a Doctrine transport

…terface (sroze)This PR was merged into the 4.3-dev branch.Discussion----------[Messenger] Fix the Doctrine transport to use the new interface| Q             | A| ------------- | ---| Branch?       | master| Bug fix?      | yes| New feature?  | no| BC breaks?    | no| Deprecations? | no| Tests pass?   | yes| Fixed tickets |#29007| License       | MIT| Doc PR        | øCommits-------75e3355 Fix the Doctrine transport to use the new interface

…uzet)This PR was submitted for the master branch but it was merged into the 4.3 branch instead (closes#10616).Discussion----------[Messenger] Describe the doctrine transportAdd documentation relative to the PRsymfony/symfony#29007TODO- [ ] Document the max granularity in seconds for delay (doctrine/dbal#2873)Commits-------27c0f9d [Messenger] Describe the doctrine tranport