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

[joostjager]

Yes, agreed on the separate PR for this. It's pre-existing.

[joostjager]

Untested ack

[joostjager]

Possible to reuse handle_bolt11_send_request matching code?

[tnull]

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.

Relatedly, I recently opened lightningdevkit/ldk-node#766 and tagged it as a BOSS topic. We could however also consider whether we want to move the CLN/LND/Eclair interop tests to the server repo, rather than doing them at the LDK Node level? Although, it is somewhat nice to do it like the other integration tests in Rust.

[tnull]

ACK, opened #114 as a follow-up.