feat: Trusted Auctioneer: implement stream execute optimistic block

[bharath-123]

This PR implements the StreamExecuteOptimisticBlock. StreamExecuteOptimisticBlock listens for optimistic block execution requests. On receiving a StreamExecuteOptimisticBlockRequest, we execute the block and wait for the mempool to be cleared. Once it's cleared, we return a StreamExecuteOptimisticBlockResponse back to the auctioneer.
We also allow only 1 client to be connected to StreamExecuteOptimisticBlock. We achieve this by maintaining an atomic boolean called executeOptimisticBlockStreamConnected where we perform a CompareAndSwap to update it from false to true everytime somebody connects to it.

Changes made:

1. Implement the StreamExecuteOptimisticBlock grpc method
2. Add mock_grpc_stream.go file which contains a basic MockStream implementation used to mock grpc streams during unit testing.
3. Add unit tests