commit-graph: add dedicated GRPC for commit-graph

Aim to address #2680 (closed)

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Commit graph is a metadata file that store the topology of a git repository commits.

Git/Gitlab relies on commit-graph to speed up various operations (i.e. git log) and without it, service degraded badly

Currently Gitlab mostly relies on git -c gc.writeCommitGraph gc to update commit-graph file. Problem arise when you want to update the commit-graph in a different granularity vs garbage collection. This becomes more relevant with the upcoming Bloom filter feature added in commit-graph format.

In this patch, I introduce the initial implementation of basic git commit-graph write call via GRPC so that we can start decouple this logic away from git gc.