Relayer API: get the contents of a queue

[tkporter]

Problem

• We have some better metrics to know broad strokes details on what the contents of the prep queue looks like, but digging into specifics (e.g. who are the senders/receivers? message content? how many retries?) still require trawling through logs

Solution

• An API that dumps the contents of the queue - maybe paginated?

[daniel-savu]

partially done in https://github.com/hyperlane-xyz/hyperlane-monorepo/pull/4089