gpg-interface: add function for converting trust level to string

gpg-interface: add function for converting trust level to string

Add new helper function `gpg_trust_level_to_str()` which will
convert a given member of `enum signature_trust_level` to its
corresponding string (in lowercase). For example, `TRUST_ULTIMATE`
will yield the string "ultimate".

This will abstract out some code in `pretty.c` relating to gpg
signature trust levels.

Mentored-by: Christian Couder <redacted>
Mentored-by: Hariom Verma <redacted>
Signed-off-by: Jaydeep Das <redacted>
Signed-off-by: Junio C Hamano <redacted>