Make a `ForkChoosingBridge`

[edmundedgar]

Currently we have the ForkableBridge but this changes under you.

Make a single contract that keeps track of the forking bridges and chooses based on a (configurable) ForkChoiceGovernor address. This in turn can do something like look at the auction result, or any other process.

Use this to abstract away the multiple bridges so you can do "send ETH to Backstop" or "get ETH back from Backstop".