Add SET command for run-time configuration of keyset_id

[tobyhede]

The Proxy should expose a SET command that enables run-time configuration of a keyset_id:

SET CIPHERSTASH.KEYSET_ID = 'EXAMPLE-UUID-4b38-9571-e0cf47802677'

• Parse set command
• Parse keyset as UUID if defined
• Set keyset_id if defined
• Error if set command but value cannot be parsed
• Update error docs with syntax details
• Extract zerokms initalisation
• Enable cipher initalisation to accept keyset_id
• Pass keyset_id when encrypting or decrypting
• Handle decrypt errors
• Refactor to extract error handling from frontend into common module
• Update backend to send decrypt errors as pg errors

[freshtonic]

This looks really good - nice work