This commit creates the necessary endpoints for onion messages.
Specifically, it adds the following:
- `SendOnionMessage` endpoint to send onion messages.
- `SubscribeOnionMessages` endpoint to subscribe to incoming onion
messages.
It uses the `msgmux` package to handle the onion messages.
Find and replace all nolint instances refering to the `lll` linter and
replace with `ll` which is the name of our custom version of the `lll`
linter which can be used to ignore log lines during linting.
The next commit will do the configuration of the custom linter and
disable the default one.
Gijs van Dam