Add a filter to RepositoryService.SearchFilesByName

The Go module proxy (!27746 (merged)) and Go packages support (!29642 (closed)) need to query Gitaly for a list of all go.mod files in a repository. RepositoryService.SearchFilesByName can be used, but git ls-tree and thus SearchFilesByName does not support glob queries, so the entire repository contents must be listed. Since go.mod files are the only results of interest, most of the results transmitted from Gitaly are discarded. Introducing filtering to SearchFilesByName would reduce unnecessary data transmission.

#218083 (comment 347686649):

For a given ref, scan for all files matching a pattern. A possible implementation would be RepositoryService.SearchFilesByName (aka git ls-tree --full-tree --name-status -r <ref> <path>) plus filtering the output. Or maybe an additional optional field for SearchFilesByName that will apply a regex or glob to the output. For Go, I would search for **/go.mod. git ls-tree allows some patterns, but not in a way that is useful to me.

This issue corresponds to Rails support for gitaly#2818 (closed).