add `authToken` to request metadata (#54115)

Adds `authToken` to the `RequestMetadata` type returned by `ctx.meta.getRequestMetadata()`.
When a request is authenticated with a user JWT (OIDC or custom), `authToken` contains that raw JWT string. For unauthenticated requests or those authenticated with an admin key, `authToken` is `null`.

This is the same token that `ctx.auth.getUserIdentity()` derives its attributes from, giving functions direct access to the raw credential when needed, e.g. to make authenticated calls to 3rd party services.

GitOrigin-RevId: d9ebf3ad67bde935d251ecdc628c04511ecbc5a2