(screenshot includes frontend changes that will be landing in a separate pull request, because this one is already big enough 😄)

Part of#10109

• Migrateexternal_auth_providers column in the database, in order to be able to contain metadata about providers, such as settings/attributes defined in Terraform.
• Add support forexternal_auth_provider.optional in the Terraform provisioner and provisionerd
• Addoptional toTemplateVersionExternalAuth
  • and fill it with new database goodness when querying/templateversions/{templateversion}/external-auth

TODO

• who usesproto.CurrentVersion? there are some tests failing because of this, and I'm not really sure what to do about it
  • also it doesn't seem like there's any machinery for checking the terraform provider version? kind of unfortunate that you could just start using 0.16 with an old version of coder andoptional would just silently disappear