[rust][client][auth] token source option

[nicolas-vivot]

This PR includes changes to improve the integration with token source / token source provider for the authentication part.
It addresses the following feature request.

The changes includes:

• the addition of a new option supportTokenSource
• the use of google-cloud-token as it already provides simple trait definition for TokenSource
• the new option replace any other similar auth configuration (ApiToken, OAuth, Bearer, Basic) since it does not make sense to have all together. (please let me know if this assumption is incorrect)

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[nicolas-vivot]

cc committee: @frol @farcaller @richardwhiuk @paladinzh @jacob-pro

[nicolas-vivot]

For now i think it's fine to depends on this crate. It is lightweight and have just a few trait definition, including the one we are interested in: TokenSource.
I plan to ask the actual maintainer of this crate to make it vendor agnostic with a new create and then depends on it.
That way, people will be free to use google-cloud-auth with the TokenProvider they need, or another cloud vendor if such crate is made / integrate with the same definition in the future.

[wing328]

thanks for the PR. let's give it a try