Allow UIDs to be managed

[neuhalje]

Is your feature request related to a problem? Please describe.
GPG keys can carry multiple user IDs. BouncyGPGs key generation currently only supports a single UID per keyring.

Describe the solution you'd like
The API for keyring management should enable the caller to

• Add user IDs to a key
• Enumerate all user IDs of a key
• Remove user IDs from a key
• Set the validity of user IDs

Describe alternatives you've considered
N/A

Additional context
E.g. this is the key of the gpgtools project:

pub   dsa2048/0x76D78F0500D026C4 2010-08-19 [SC] [expires: 2020-06-15]
      85E38F69046B44C1EC9FB07B76D78F0500D026C4
uid                   [ultimate] GPGTools Team <team@gpgtools.org>
uid                   [ultimate] GPGMail Project Team (Official OpenPGP Key) <gpgmail-devel@lists.gpgmail.org>
uid                   [ultimate] GPGTools Project Team (Official OpenPGP Key) <gpgtools-org@lists.gpgtools.org>
uid                   [ultimate] [jpeg image of size 5871]
sub   elg2048/0x07EAE49ADBCBE671 2010-08-19 [E] [expires: 2020-06-15]
sub   rsa4096/0xE8A664480D9E43F5 2014-04-08 [S] [expires: 2024-01-02]

[dsanghan]

@neuhalje Did you manage to figure out how to add a user ID to an existing key?