Adapt Teleporter contracts for use with AvalancheValidatorSetRegistry

[michaelkaplan13]

Adapt and/or extend the existing Teleporter contracts to be able to be used in tandem with an AvalancheValidatorSetRegistry for receiving messages from Avalanche chains on external EVM chains.

The "send" functions can be left with temporary stub implementations until the approach for sending messages to Avalanche from external EVM chains is decided on.

If done correctly, these modifications should allow for contracts built on top of Teleporter (such as ICTT) to be used on external chains with no further modifications.