This adds support for constructing aServiceAccount directly from aServiceAccountKey, rather than from a file. It also:

• Adds interior mutability using a tokioRwLock to enable cloningServiceAccount while re-using the current access token across threads/instances. Also allowsServiceAccount to be used without needing to bemut.
• MakesServiceAccountKey and its fields public to allow parsing or constructing the struct from any source (e.g. string, bytes, etc.) and also allows the fields inside to be re-used by other modules (e.g. readingproject_id necessary for FCM)
• AddsServiceAccountBuilder to enable the use of a variety of constructors, including the ability to re-usereqwest::Client across instantiations which enables sharing connection pools. Existingfrom_file() anduser_email() methods continue to work for backwards-compatibility, but the builder pattern is generally a better alternative
• access_token() now returnsAccessToken instead of e.g."Bearer token" which allows the use of reqwest's.bearer_token() method without needing to parse-out the token. Also enables the invoker to see when the token will expire.
• Asserts that the returned token is indeed a bearer token
• from_file() retained as a helper method and now returns aResult type since file reading and parsing is performed in this function, rather than lazily inaccess_token(). Also now accepts any path-compatible value, rather than just&str.
• StoresRsaKeyPair inJwtToken rather than inputs to improve efficiency
• Catch and re-throw any non-200 status codes when requesting an access token. Also generally improve error handling by returning the original error, rather than converting into a string.
• Resolves various clippy errors

Resolves#28