fetch-pack: move write_fetch_command_and_capabilities() to connect.c

fetch-pack: move write_fetch_command_and_capabilities() to connect.c

In a subsequent commit write_fetch_command_and_capabilities() will be
refactored to a more general-purpose function, making it more accessible
to additional commands in the future.

Move write_fetch_command_and_capabilities() to 'connect.c', where
there are similar purpose functions.

Because string_list is only used as a pointer, use a forward
declaration [1].

[1]: https://lore.kernel.org/git/Z0RIqUAoEob8lGfM@pks.im/

Helped-by: Jonathan Tan <redacted>
Helped-by: Christian Couder <redacted>
Signed-off-by: Calvin Wan <redacted>
Signed-off-by: Eric Ju <redacted>
Signed-off-by: Pablo Sabater <redacted>
Signed-off-by: Junio C Hamano <redacted>