Improve default branch option support for Project API

Follow-up for #26261 (closed)

!63771 (merged) adds basic support for default_branch parameter. It currently has a limitation, that repository must be initialized in order to apply the requested branch name.

Based on the suggestion we can define the initial branch even for the empty repository.

But gitaly does not support this option yet.

Proposal

1. Extend CreateRepository request with a new option default_branch and apply its value as a flag to git init command (--initial-branch=<default_branch>) - gitaly!3582 (closed)
2. Update gitaly client to support default_branch and allow to use default_branch option separately from initialize_from_readme