remote: move remote group resolution to remote.c

remote: move remote group resolution to remote.c

`get_remote_group`, `add_remote_or_group`, and the `remote_group_data`
struct are currently defined as static helpers inside builtin/fetch.c.
They implement generic remote group resolution that is not specific to
fetch — they parse `remotes.<name>` config entries and resolve a name
to either a list of group members or a single configured remote.

Move them to remote.c and declare them in remote.h so that other
builtins can use the same logic without duplication.

Useful for the next patch.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>