The `cln-plugin` can be used to create a plugin that registers
additional rpc-methods. However, it doesn't allow to specify the `usage`
of the command.
This change makes it possible to specify the `usage`. It should not
contain any breaking changes.
I've opted to add a new method called `rpcmethod_from_builder` to the
`Builder` struct. This approach allows us to add new parameters in the
future.
0d877ffb71