x/build/cmd/gomote: add authentication component

[cagedmantis]

Add a component which will handle the Identity Aware Proxy validation. If possible, it should be extensible for possible future use in the build infrastructure.

This is a component of the project to revamp the security model used by gomote #47521
@golang/release

[gopherbot]

Change https://golang.org/cl/358915 mentions this issue: internal/access: add access package

[gopherbot]

Change https://golang.org/cl/371396 mentions this issue: internal/access: add helper functions for testing authentication