Add Bitcoin Core REST as a chain-source option #280

Context

ldk-node supports a fourth chain-data-source option — set_chain_source_bitcoind_rest(),
backed by BitcoindRestClientConfig — alongside the RPC, Electrum, and Esplora sources
ldk-server already exposes via ChainSource (ldk-server/src/util/config.rs). ldk-server
currently has no REST variant.

Why this is worth adding

• Bitcoin Core's REST interface serves block/header/tx data unauthenticated by default
  (no rpcuser/rpcpassword needed for reads), which simplifies self-hosted setups where
  operators don't want to expose or manage full RPC credentials.
• It's a lighter-weight, cacheable path for block/header retrieval compared to JSON-RPC.

Maturity check

Two correctness bugs in ldk-node's REST chain source were found and fixed recently:

• bitcoind: fix raw TX fetching when using REST ldk-node#1006
• Follow reorgs with bitcoind REST ldk-node#1022
• rest: Allow fetching stale header bitcoin/bitcoin#35909

One known non-blocking edge case remains: a narrow startup race where sync_wallets could
run before initial sync registers the SPV client. It's mitigated by the RPC fallback and
was deliberately deferred by the ldk-node maintainer rather than blocking the reorg fix.

Happy to wait for an 0.8 release to depend on if maintainers would prefer that over pinning to a
main revision.

Proposed scope

• Add a Rest { rest_host, rest_port } (or similar) variant to ChainSource in config.rs
• Wire it to set_chain_source_bitcoind_rest() in main.rs
• Add [bitcoind_rest]-style config parsing to ldk-server-config.toml
• Docs + a CI fixture (bitcoind with -rest=1), following the existing bitcoind/Esplora
  e2e test setup

Open to feedback on the config shape.