List signing services by type #779

Summary

When using signing services it is essential to use the correct type of signing service.
Using AsciiArmoredDetachedSigningService where the plugin expects AptReleaseSigningService won't work.

However, the pulp signing-service command does not appear to know a --type option.

Examples

I want to be able to do something like:

pulp signing-service --type AptReleaseSigningService list

To list just the signing services of the specified type.

Challenges

I am not sure how problematic it is that pulp signing-service is a pulpcore associated command, while many signing service types may be plugin specific.