profilestore: Add unimplemented WriteArrow RPC (#6214)

* profilestore: Add unimplemented WriteArrow RPC

Add a new WriteArrow unary RPC to the ProfileStoreService that accepts
an Arrow IPC buffer. The implementation reads the IPC buffer and logs
the number of rows per record batch at debug level, then returns
Unimplemented to the client.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* signedrequests: Ensure credential type is serviceaccount

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: brancz