Extract out keystore code to librekey

[ronaldtse]

Description

Keystore handling code should be extracted out to librekey. Need a keystore interface for interacting with keystore operations.

Keystore operations:

• Store key
• Fetch/Find key
• Edit key
• Key metadata / properties
• Maybe some trust related data (as metadata?)

rnp should depend on librekey and utilize the general keystore interface to interact with all keystores.

This includes:

• PGP keyring
• g10 key directory (private-keys.v1.d)
• KBX format
• SSH keychain

[ni4]

@ronaldtse @dewyatt Should we close this since we ended up with the single library?

[ronaldtse]

@ni4 a shame we weren't able to do generalize the keystone mechanisms... Do you think this will remain useful for the future?

[ni4]

@ronaldtse Actually it's not a shame but impossibility to separate key packets/other packets loading/parsing/composition. The only way I see is to have separate libs for keyring format(s) which are dependent on base lib, where base lib operates with only OpenPGP packet sequences (transferable public/secret keys). So g10/kbx code can be separated, but it still will use base library's code to build up key structures.