Fix: blockchain.transaction.id_from_pos compatibility with electrs

[jharveyb]

While working on a lightning GossipVerifier in ldk-node, I ran into an issue with the Electrum client.

tl;dr electrs diverges from the Electrum protocol a bit, such that the blockchain.transaction.id_from_pos method doesn't work right now. Fixing this is pretty simple.

There are two issues: the 3rd parameter, merkle, not being treated as optional, and the return value formatting when merkle = false.

To reproduce, you can run the relevant test against an electrs/0.11.1 server found on https://1209k.com/bitcoin-eye/ele.php, (not qtornado, which is ElectrumX):

TEST_ELECTRUM_SERVER=electrum.tjader.xyz:50002 cargo test raw_client::test::test_txid_from_pos -- --exact

Finished `test` profile [unoptimized + debuginfo] target(s) in 0.04s
     Running unittests src/lib.rs (target/debug/deps/electrum_client-e59677bf712d816c)

running 1 test
test raw_client::test::test_txid_from_pos ... FAILED

failures:

---- raw_client::test::test_txid_from_pos stdout ----

thread 'raw_client::test::test_txid_from_pos' (3145567) panicked at src/raw_client.rs:1965:53:
called `Result::unwrap()` on an `Err` value: Protocol(Object {"code": Number(-32602), "message": String("invalid params")})
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

If the library adds the merkle = false parameter, there is still an issue with parsing the response:

running 1 test
test raw_client::test::test_txid_from_pos ... FAILED

failures:

---- raw_client::test::test_txid_from_pos stdout ----

thread 'raw_client::test::test_txid_from_pos' (3147516) panicked at src/raw_client.rs:1965:53:
called `Result::unwrap()` on an `Err` value: JSON(Error("invalid type: map, expected an ASCII hex string", line: 0, column: 0))
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

You can see the electrs behavior here:

https://github.com/romanz/electrs/blob/9cd09afa229df89214f59816e5142f0f58ff6c48/src/electrum.rs#L466

Additionally, the JSON map key was tx_id before version 0.11.1:

https://github.com/romanz/electrs/blob/800bebeea4a3ec4ca924b389338b3449c81b28d4/src/electrum.rs#L484

This PR accepts both versions. You can confirm that behavior with a public electrs v0.10.9 server (I didn't get this working with the cargo test runner):

Terminal 1: `ncat --sh-exec "ncat -k --proxy-type socks5 --proxy 127.0.0.1:9050 4vrz2q62yxlfmcntnotzdjahpqh2joirp2vrcdsayyioxthffimbp2ad.onion 50001 -v" -l 50506 -k`

Terminal 2: `echo '{"jsonrpc": "2.0", "method": "blockchain.transaction.id_from_pos", "params": [932453,139,false], "id": 0}' | netcat 127.0.0.1 50506`

{"id":0,"jsonrpc":"2.0","result":{"tx_id":"8fed750149034b9e03d5143a95aaa52ee54aa231564f6519be754cfacee0cd0d"}}

Added tests for the parsing, and the raw_client:

TEST_ELECTRUM_SERVER=electrum.tjader.xyz:50002 cargo test raw_client::test::test_txid_from_pos -- --exact
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.06s
     Running unittests src/lib.rs (target/debug/deps/electrum_client-e59677bf712d816c)

running 1 test
test raw_client::test::test_txid_from_pos ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 63 filtered out; finished in 0.77s

Everything is passing locally for me, though that electrum.tjader.xyz server seems finicky. I also tested this branch in my electrum CLI and confirmed it works against both my personal electrs server, and electrum.blockstream.info.

Something like https://github.com/testcontainers/testcontainers-rs could be useful for testing against multiple impls if there's no public (reliable) electrs instance.