Add missing methods for OpenPGP v6 API

[vanitasvitae]

This PR contains some missing methods that were not yet merged in #1911

The PR was created by adding in missing functionality that the current v6 branch of PGPainless depends on, so it is not complete, but adds enough to make PGPainless compile and allow the tests to pass.

Hope this helps :)

[vanitasvitae]

Some background on c931f24 :
I'd like to keep this a (functional) interface if possible. The factory methods can be moved into another class (as I did -> Util), but keeping the class an interface enables implementers to simply provide a lambda expression when a KeyPairGeneratorCallback object is required.

[vanitasvitae]

Do you have any plans on when this PR can be merged? I'd like to make a new PGPainless release using the new v6 API at some point but the missing changes already blocked me from making a release using BC 1.81, which was a bit unfortunate.

[ligefeiBouncycastle]

@vanitasvitae Thanks for the update! The final reviewer is currently on leave and will be back next Monday. In the meantime, I plan to push the branch into the final review phase by this Friday, so it’ll be ready for them upon return.

Appreciate your patience. I’ll do what I can to keep things moving.