feat: key not from file & other improvements

[chris13524]

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

• Adds interior mutability using a tokio RwLock to enable cloning ServiceAccount while re-using the current access token across threads/instances. Also allows ServiceAccount to be used without needing to be mut.
• Makes ServiceAccountKey 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. reading project_id necessary for FCM)
• Adds ServiceAccountBuilder to enable the use of a variety of constructors, including the ability to re-use reqwest::Client across instantiations which enables sharing connection pools. Existing from_file() and user_email() methods continue to work for backwards-compatibility, but the builder pattern is generally a better alternative
• access_token() now returns AccessToken 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 a Result type since file reading and parsing is performed in this function, rather than lazily in access_token(). Also now accepts any path-compatible value, rather than just &str.
• Stores RsaKeyPair in JwtToken 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

[makarski]

Hey @chris13524. Thanks for the PR. I managed to spot a few minor things, hope this is not too much of a hustle to do the changes. Also would be great to bump up the version in Cargo to 0.9, since there is a breaking change to access_token return type.

[makarski]

std::result::Result could be shorted to StdResult, since there is already an import on line 3

[makarski]

Looks like there is a typo in Beaarer

[makarski]

This piece seems not to compile for me. Here is the error message that I am get

error[E0015]: cannot call non-const fn `chrono::Duration::seconds` in constants
   --> src/serv_account/mod.rs:108:34
    |
108 |         const LEEWAY: Duration = Duration::seconds(30);
    |                                  ^^^^^^^^^^^^^^^^^^^^^
    |
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
    ```

[makarski]

One of the options to resolve the issue with Duration above would be to keep it simple and use ints:
Utc::now().timestamp() as u64 + json.expires_in - 30

[chris13524]

Thanks for the review! I'm working on some fixes for the above as well as some other tweaks