Add support for a query param to gitaly's FindCommits RPC

Context

In an effort to improve the Commit List page with additional abilities, we're looking to expand the functionality of the gitaly RPC: FindCommits with a query parameter to allow text search (current feature).

Previous spike's conclusion:

The FindCommitsRequest supports a wider range of arguments but not a query term. Looking at the gitaly code I believe it's possible to add this relatively easily as the FindCommits RPC is using git log to find the commits which is also how the CommitsByMessage RPC works with the addition of a --grep flag when calling git log.

— @jwoodwardgl in #535989 (comment 2501899396)

Scope of this issue

This issue aims at both:

1. confirming the viability of this path and identify potential unidentified risks (reaching out to gitaly experts)
2. if confirmed, proceed to executing the change