[HotWallet] Add txsender

[ian-shim]

This PR introduces a new interface txsender, which supports a few methods:

• SendTransaction: signs a raw transaction and broadcasts it to the network
• GetTransactionReceipt : retrieves the receipt if available
• SenderAddress: address of the transaction sender

It also refactors the TxManager to use txsender instead of signing/sending the transaction itself.

This interface will be used to implement remote MPC txsender that uses Fireblocks API.

[samlaf]

So we're left with these 2 interfaces

type TxSender interface {
	SendTransaction(ctx context.Context, tx *types.Transaction) (TxID, error)
	WaitForTransactionReceipt(ctx context.Context, txID TxID) (*types.Receipt, error)
	SenderAddress() common.Address
}

type TxManager interface {
	Send(ctx context.Context, tx *types.Transaction) (*types.Receipt, error)
	GetNoSendTxOpts() (*bind.TransactOpts, error)
}

which look very very similar to me. Do we still need the txmgr? Unclear to me that they don't serve the same purpose?

[ian-shim]

which look very very similar to me. Do we still need the txmgr? Unclear to me that they don't serve the same purpose?

I agree that they look very similar from the interface/naming. Maybe we can clarify the naming of the two interfaces, but my understanding is the two have different purposes. txmgr "manages" transactions in that it would manage the nonces, regas logic in case tx is stalled, etc. while txsender supports more primitive functionalities like sending a raw transaction. So txmgr would use txsender to achieve the high level functionalities

[samlaf]

LGTM!