`gix-protocol::fetch()` without Delegates

[Byron]

Having learned from gix it should now be possible to have a command-oriented fetch that is similar to gix_protocol::ls_refs().

Motivated by #1319, as these changes should allow heartwood to use most of gix-protocol without duplicating a lot of code.

Tasks

• [ ] make current fetch() test-only
• [ ] re-create fetch() as command similar to ls_refs(), based on gix
• [ ] re-implement the test-only fetch with the command-oriented fetch, maybe provide it as utility in gix-protocol even? That would allow the Delegates to remain, but be specific to this particularly 'simple' way of performing a fetch.
• [ ] let gix use the command-oriented fetch.