Add LibrePGP PreferredEncryptionModes signature subpacket

[vanitasvitae]

I decided to implement this in its own dedicated class to express that this is not an official OpenPGP signature subpacket.

[vanitasvitae]

This PR now includes changes from #1464

Edit: The codebase still contains these two methods, which are broken:

• https://github.com/bcgit/bc-java/pull/1743/files#diff-a2c092a35f74b59254a2567f79afdf94b52e620a972835302375ef63af4ce165R196-R210
• https://github.com/bcgit/bc-java/pull/1743/files#diff-3104d50400a1636fcc1f441c9703b57a08266042db8962a3fd8809f5b9945a71R290-R307

These methods take/return a single int[] as algorithms and store/retrieve those in a signature subpacket with packet ID 39 (OpenPGP).
This is a mix of LibrePGP- and OpenPGP-style AEAD preferences.

For now I have marked these methods as broken and deprecated, but really I'd rather remove them completely.
What do you think?