auth-server: add gas cost comparison

[sehyunc]

Purpose

This PR adds methods to fetch gas price and ETH/USDC rate to estimate the gas cost of executing a quote.

Testing

• Tested locally
• Test in testnet

[akirillo]

have some questions & a couple nits, but nothing blocking

[akirillo]

First time I'm looking at this & it's out of scope for this PR / review, but why do we hold references (&'a QuoteResponse) instead of owned types (QuoteResponse) here? Generally find this to make things much less ergonomic due to managing lifetimes.

QuoteResponse is a pretty lightweight struct, it should be totally fine to clone it into QuoteComparison if ownership can't be taken directly

[akirillo]

Looks like this file was copied over from the fee sweeper, which is sensible. However, since we're just relying on the price report route, we should go direct to the price reporter & just request the USDT-quoted price. This will take load off of the relayer, and since it's not the source of truth for prices it's unclear that we should continue to support the price report route on it. It should be simple enough to build an HTTP client for it (or even a single helper method for fetching a price)