Add APIs for CRUD actions on groups

Currently you can implicitly create a group within a private message send, by specifying the group members and name in-line in the message itself. This will hash to ensure the same group is re-used when the same list of recipients is specified, with the same name, on the same ledger (on any member of the group).

However, it should be possible to create the groups explicitly, and it definitely needs to be possible to query the state of groups.

There's also potentially sub-state filtered by groups that might be very useful. Specifically:
- The `contexts` that have history on that group, and the `nexthashes` of each
- The sequence of `pins` for that group

... ideally it should be straightforward to use a set of instructions, to work out if a group is blocked, and hopefully work out which message is blocking a group.