Add Transaction Fee RPC to Statemint/Statemine

[shawntabrizi]

Client needs to expose the RPC for this.

1. impl_runtime_apis: https://github.com/paritytech/polkadot/blob/master/runtime/polkadot/src/lib.rs#L1373

2. Extract rpc stuff into its own module from: https://github.com/paritytech/cumulus/blob/master/polkadot-parachains/src/service.rs

   • look at links below from polkadot and substrate

3. Reconnect via RPC to service and expose the new rpcs:

   • https://github.com/paritytech/polkadot/blob/master/rpc/src/lib.rs#L111
   • https://github.com/paritytech/polkadot/blob/master/rpc/src/lib.rs#L152

Some notes on extending RPC here: https://github.com/paritytech/substrate/blob/master/bin/node-template/node/src/rpc.rs#L58

[Swader]

@hamidra do you need help with this? This is kind of important for us and we'd love to try and help, but if it's something that's already in progress we'll hold off

[hamidra]

Hey @Swader. Changes seems to be straightforward. Will work on it today to send a PR.

[apopiak]

PR is merged