Add support for ObjectFormatRequest RPC

What does this MR do and why?

Contributes to #426803 (closed)

Problem

There is no way to fetch object format for existing repositories.

Solution

Use ObjectFormatRequest RPC to detect repository object format.

How to set up and validate locally

1. Open Rails console
2. Execute Project.last.repository.object_format
3. This code will make a call to Gitaly RPC and return a result (sha1)

MR acceptance checklist

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