add minisign support

[andreasstieger]

Adds support for verifying minisign based signatures. Keeps the pattern of the GnuPG part of the script of using .keyring files, but looks for *.minisig files.

Testing done:

1. still validates OpenPGP signatures:

$ osc co devel:tools:building/cmake
$ cd devel:tools:building/cmake
$ osc service localrun source_validator

2. graceful exit when installing but minisign is not installed

$ ls -1
ccache-4.12.2.tar.xz
ccache-4.12.2.tar.xz.minisig
ccache.changes
ccache.keyring
ccache.spec

$ osc service localrun source_validator
WARNING: Command 'localrun' is obsolete, please use 'run' instead.
Running source_service 'source_validator' ...
/usr/lib/obs/service/source_validators/25-keyring-validate: line 34: minisign: command not found
ERROR: signature [...]/ccache/ccache-4.12.2.tar.xz.minisig does not validate
Aborting: service call failed:  /usr/lib/obs/service/source_validator --outdir [...]/ccache/tmpa67x09de.source_validator.service

3. validation

$ zypper in minisign
$ osc service localrun source_validator
WARNING: Command 'localrun' is obsolete, please use 'run' instead.
Running source_service 'source_validator' ...

4. verbose, dropping -q:

Signature and comment signature verified
Trusted comment: timestamp:1764182754   file:ccache-4.12.2.tar.xz       hashed

5. with missing minisign command:

Running source_service 'source_validator' ...
ERROR: minisign command not available
Aborting: service call failed: [...] 

[andreasstieger]

Working on getting minisign into the distro first.. boo#1256939

[bugfinder]

looks good to me