Ability to obtain current LSP's trampoline fee rate

[JssDWt]

In the trampoline protocol between greenlight and breez LSPs, there's currently no way to learn the LSP's routing policy for trampoline payments. It is set in agreement and enforcement, but not in messages.

Add an api/api field containing the trampoline node's routing policy.

Prerequisites:

• communicate the trampoline policy to the node in a custom message after init
• read the custom message on the sdk side (greenlight side?) to learn the policy
• store the policy in-memory during the connection session and expose it through an api.