ref-filter: add new "signature" atom

ref-filter: add new "signature" atom

Duplicate the code for outputting the signature and its other
parameters for commits and tags in ref-filter from pretty. In the
future, this will help in getting rid of the current duplicate
implementations of such logic everywhere, when ref-filter can do
everything that pretty is doing.

The new atom "signature" and its friends are equivalent to the existing
pretty formats as follows:

	%(signature) = %GG
	%(signature:grade) = %G?
	%(siganture:signer) = %GS
	%(signature:key) = %GK
	%(signature:fingerprint) = %GF
	%(signature:primarykeyfingerprint) = %GP
	%(signature:trustlevel) = %GT

Co-authored-by: Hariom Verma <hariom18599@gmail.com>
Co-authored-by: Jaydeep Das <jaydeepjd.8914@gmail.com>
Co-authored-by: Nsengiyumva Wilberforce <nsengiyumvawilberforce@gmail.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Kousik Sanagavarapu <five231003@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>