should we expose bundle & block timestamps?

Idea: expose three new timestamps in `/transactions` and `/blocks`

* `block.witnessed_at`
* `transaction.received_at`
* `transaction.sent_to_miners_at`

@jiangjinyuan already made an issue about block timetamps: https://github.com/flashbots/mev-blocks/issues/7

Returning the timestamp at which a new block was witnessed by the relay will let you know exactly how much time you have, between then and when the block gets mined, to submit a bundle.

I think this is an easy yes.

Exposing data about other searchers' bundles, however, might be contentious.

**Pros:**
* allows searchers to compare timestamps of conflicting bundles
* this way you can know whether your opponent is sending bundles before you

**Con:**
* anyone can see how quickly your bot builds and sends a bundle (thereby leaking info about how fast your strategy's algorithm is)