Canonical fee-based relay decision

[cam-schultz]

Context and scope
The existing relay decision logic is solely concerned with correctness, and not profitablility. For example, the teleporter package MessageHandler implementation relays the message if 1) it is an allowed relayer and 2) the message has not been delivered already. Otherwise, it always spends the compute, networking, and gas to aggregate a Warp signature and deliver the message to the destination.

Alonside architectural changes to aid in implementing custom profitability logic, we should provide a basic example of how the relayer can parse a Teleporter message and make a profit-based decision. Since it's meant only as an example, using a free tier API should suffice.

Discussion and alternatives
We should wait until #335 to implement this, and aim to have it fit into the new archtecture.