Add from_credentials_json function.

[brocaar]

See #45.

This adds a from_credentials_json function so that the credentials can be parsed from a JSON string.

I changed get_authentication_manager such that instead of an optional path, one can pass an Option<CustomServiceAccount>. The from_credentials_file function now handles the reading of the given path and the from_credentials_json handles the reading of the given JSON.

I updated the init() function such that it first checks GOOGLE_APPLICATION_CREDENTIALS and if it exists, it will try to read from that path (if not it errors). If no such env variable exists, it calls get_authentication_manager with None.

[djc]

Thanks for working on this! Still need to clean up the clippy lint, too.

[brocaar]

Thanks for reviewing and merging @djc !

[djc]

@brocaar I made a bunch of follow-up changes in #49 which change the API for this, let me know if you have any feedback.