unstable_rpc: Add transactionBroadcast and transactionStop

[lexnv]

This PR adds the transaction_unstable_broadcast and transaction_unstable_stop APIs.

While at it, added a test to check that the broadcasted transaction gets into a finalized block.

[jsdw]

Nit: this is all good, but if we wanted we could do something like this to avoid relying on any sort of ordering etc:

let tx_hash = SubstrateConfig::Hashing::hash_of(tx_bytes);

// ...

let Some(ext) = extrinsics
    .iter()
    .find(|e| SubstrateConfig::Hashing::hash_of(e.bytes()) == tx_hash)
    .next() else { continue };

let ext = ext.as_extrinsic::<node_runtime::balances::calls::types::TransferAllowDeath>()
    .unwrap()
    .unwrap();

// ...

[lexnv]

There is just a small inconsistency with this:

• create_signed_offline returns the extrinsic bytes including the prefix of compact encoding length
• ExtrinsicDetails will strip the compact prefix:
  

  subxt/subxt/src/blocks/extrinsic_types.rs

  Line 224 in a211e98

  let bytes: Arc<[u8]> = strip_compact_prefix(extrinsic_bytes)?.1.into();

I've adjusted the code to reflect this, not sure if we should do anything else to align for ex with polkadot-api 🤔

[jsdw]

Has the PR to rename to v1 merged in Substrate already? :)

[niklasad1]

no, https://github.com/paritytech/polkadot-sdk/blob/master/substrate/client/rpc-spec-v2/src/transaction/api.rs#L50

[jsdw]

A couple of minor things but yay for more RPCs :)

When this merges we can also move our TX submitting to use this method and thus be "reconnect" safe at some point which will be nice (though much more complicated, because aside from searching blocks for the TX we need to periodically validate it to know whether to stop checking for it etc).