Add RPC functions for keysend, msg signing, and exporting scorer

[benthecarman]

This basically fills out the remaining ldk-node functions that were not used by the RPC/CLI.

[ldk-reviews-bot]

I've assigned @joostjager as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[joostjager]

One general question that I had for ldk-server is whether we want some light e2e testing? cli->client->server path.

Perhaps an LdkNode trait is a possibility to mock ldk-node and focus testing on ldk-server.

[benthecarman]

One general question that I had for ldk-server is whether we want some light e2e testing? cli->client->server path.

Yeah that would be nice, probably better for a separate PR though