[identity] Support service-to-service token in auth RPCs

Summary:
To call authenticated RPCs from other services, we have to support providing s2s token.
Modified the interceptor to look for the token, in addition to traditional Auth Metadata (uid + did + csat).

Test Plan: Tested locally, using GUI gRPC client and default localstack-stored service-to-service token.

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14465