Add gitaly_git_blob_raw feature

What does this MR do?

Add two new Gitaly migrations with feature flags:

• gitaly_git_blob_raw (Gitlab::Git::Blob.raw)
• gitaly_git_load_all_data (Gitlab::Git::Blob#load_all_data!)

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

gitaly#331 (closed)

gitaly#332 (closed)