feat: tx client v2

[mcrakhman]

Overview

This note summarizes the logic implemented in grpc/src/tx_client_v2.rs and
grpc/src/tx_client_impl.rs.

This is very roughly made according to the spec (with main ideas):
https://github.com/celestiaorg/celestia-app/blob/main/specs/src/tx_client_v2.md

tx_client_v2.rs

• Provides the generic transaction pipeline split into a front-end submitter
  (TransactionManager) and a background worker (TransactionWorker).
• TransactionManager::add_tx serializes sequence reservation, signs the
  request via a SignFn, attaches one-shot callbacks, and enqueues the signed
  transaction.
• TransactionWorker maintains an ordered queue, tracks per-node submission
  state, and drives the lifecycle with a single event loop:
  • Submits signed transactions to nodes, respecting sequence order.
  • Polls confirmation in batches (status_batch) on a timer.
  • Emits TransactionEvent results to resolve submit/confirm callbacks.
  • Handles recoverable failures by moving to a recovery state and re-syncing
    expected sequence.
• Confirmation is in-order: confirmed info is buffered by sequence and drained
  when all prior sequences are confirmed.

tx_client_impl.rs

• Implements the generic traits for a real gRPC client:
  • GrpcSignContext fetches account/chain state and gas estimates.
  • SignFnBuilder builds a SignFn<Hash, TxInfo> that creates signed tx bytes
    from TxRequest and TxConfig.
  • TxServer for GrpcClient wires submit and status_batch to gRPC calls and
    maps failures into v2 error types.
• Adds TransactionService which hides the worker plumbing:
  • Builds a TransactionWorker using the client signer and current sequence.
  • Exposes submit(TxRequest, TxConfig) -> TxHandle<Hash, TxInfo>.
  • Supports recreate_worker() to rebuild the worker and manager if needed.