Expose signature author email from Gitaly

What does this MR do and why?

Since gitaly!7166 (merged), author email is returned when using the GetCommitSignatures RPC.

This MR exposes the signature author email for use in !171259 (merged).

References

Broken down from !170444 (closed).
Unblocks !171259 (merged).
Related to #498669 and #425042.

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