Add separate EL endpoint for submitting TXs #182
Description
Right now HD submits all of its transactions against its own Execution Client. This is problematic when trying to send TX's while syncing, and doesn't take advantage of side-channel APIs like Flashbots Protect that offer revert protection to save on gas. It needs a separate config setting for a custom EC URL that can be used for sending transactions (while using the local EC as a secondary if it fails). Note that queries probably should not be done on this endpoint because of potential rate limiting issues; it should only be used for submitting transactions.