[RFC] ref-filter: add new atom `signature`

[Hariom verma <hariom18599@gmail.com>]

Hi,

In ref-filter, we have a bunch of atoms which serves as a formatting
option for `git for-each-ref`.

Despite of having 'subject' as an argument to 'contents' [i.e
`%(contents:subject)`], we still have 'subject' as an atom [i.e
`%(subject)`].
Likewise for `%(contents:signature)`, we can have 'signature' as an atom too.

Currently, `%(contents:signature)` internally uses `parse_signature()`
that only works for signed tag objects. I plan to expand the scope of
`signature` by defining it as an atom and will make it work for
commits too.
Also, thinking of adding `signer`, `key`, `fingerprint`,
`primarykeyfingerprint`, `trustlevel` and `grade`[print 'G' for good,
'B' for bad...] as arguments to "signature".

This change will also help me in using ref-filter's logic in pretty.c
for formatting options like "GG", "G?", etc. To know more about these
formatting options, you can take a look here[1].

If curious about what approach I'm taking for using ref-filter logic
in pretty.c, see[2]

Would love hear your thoughts on it. Any suggestions will also be appreciated.

Thanks,
Hariom

[1]: https://github.com/git/git/blob/f402ea68166bd77f09b176c96005ac7f8886e14b/Documentation/pretty-formats.txt#L221-L235
[2]: https://public-inbox.org/git/pull.658.git.1592218662.gitgitgadget@gmail.com/