Followup: Lookup real features for LDK-server

**Describe the followup**

From #307, blocked on ldk-server change.

We [currently hardcode](https://github.com/bitcoin-dev-project/sim-ln/pull/307/changes#diff-55302799864abb244abbbeb3ef2bbc3d9155ce8b7102bae46a4782e3e0d53a95R182) nodes features in LDK-server because they're not surfaced. Once https://github.com/lightningdevkit/ldk-server/pull/237 is in, we can update sim-ln to use the actual node's features.

Really shouldn't be an issue unless somebody turns off keysend, but good to be correct.

Start at the hardcoded node-feature handling shown around line 182 in the changes for #307, after confirming that ldk-server PR #237 has landed. Replace the hardcoded features with the actual node's surfaced features, and verify that sim-ln uses the real feature state rather than the fixed values.

Written by the indexing model from the issue text.