From: Nsengiyumva Wilberforce <nsengiyumvawilberforce@gmail.com>

This commit duplicates the code for `signature` atom from pretty.c
to ref-filter.c. This feature will help to get rid of current duplicate
implementation of `signature` atom when unifying implemenations by
using ref-filter logic everywhere when ref-filter can do everything
pretty is doing.

Add "signature" atom with `grade`, `signer`, `key`,
`fingerprint`, `primarykeyfingerprint`, `trustlevel` as arguments.
This code and its documentation are inspired by how the %GG, %G?,
%GS, %GK, %GF, %GP, and %GT pretty formats were implemented.

Co-authored-by: Hariom Verma <hariom18599@gmail.com>
Co-authored-by: Jaydeep Das <jaydeepjd.8914@gmail.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Nsengiyumva Wilberforce <nsengiyumvawilberforce@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.1428.git.git.1673254961028.gitgitgadget@gmail.com