auth: expose AccessToken expiration #7410

Is your feature request related to a problem? Please describe.
I have a short-lived CLI that needs an access token on every run. It keeps the token in a private file between runs (much like gcloud's own token cache), so it needs to know when the token expires.

AccessToken only exposes token. The SDK tracks the expiry internally but drops it in the conversion, so the CLI can't get the lifetime from the SDK. Today it gets tokens from gcloud auth print-access-token and looks up the lifetime with the tokeninfo endpoint after each fetch.

Describe the solution you'd like
A way to get the expiration along with the token, or None when it isn't known. This was part of the original sketch in #2929, and #7391 implements it as a field. An Instant is fine for my use; I'd convert it to wall-clock time when saving.

One semver note: AccessToken has only public fields and isn't #[non_exhaustive], so adding any field, even a private one behind an accessor, breaks struct-literal construction (for example in mocks of AccessTokenCredentialsProvider). cargo-semver-checks reports that as major (constructible_struct_adds_private_field). A new method that returns a #[non_exhaustive] type with the token and its expiry would leave AccessToken unchanged.

The other Google auth libraries already expose this:

• Go: auth.Token.Expiry
• Java: AccessToken.getExpirationTime()
• Python: Credentials.expiry

Describe alternatives you've considered

• Looking up tokeninfo after each fetch (current workaround): an extra request per token.
• Assuming a one-hour lifetime: unsafe, since lifetimes vary by credential type.
• Fetching a fresh token in every process: works, but adds a token request to every run.

Additional context
On #7391 there was a question of whether this needs to be exposed at all. This is a case where the SDK's in-process cache can't help: the process exits before the token could be reused.