Add IgnoreRevisionsBlob field to RawBlameRequest

Add a IgnoreRevisionsBlob field to RawBlameRequest to allow gitaly to find a blob that can be used as the file for the --ignore-revs-file flag in git blame

Also update the RawBlame response to include appropriate errors when the blob is invalid (e.g contains non-existent objects)

There is some existing work in progress here: gitaly!4177 (closed)