Customizable gossipsub backoff on unsubscribe

[Menduist]

This is an alternative to status-im/nimbus-eth2#3025

It allows applications to choose a different backoff when we unsubcribe from a gossipsub topic.

Currently, if you unsubscribe - resubscribe from a topic in less than backoff seconds, all of your peers that were previously in your mesh are in your backoff, and won't allow you to graft them back. Setting the unsubcribeBackoff to a lower value allows you to alleviate this issue.

Note that it should still be >0 seconds, or some implementations (including ours) will ignore it. However, if it's less than BackoffSlackTime (2 seconds), it will effectively be a 0 seconds backoff.
It should not be an issue to have a 0 seconds backoff, since you're unsubscribed and peers should not attempt to graft you until you resub anyway. But maybe to avoid "race conditions", it's a good idea to keep a few seconds as backoff, just in case. Worst case scenario, a peer may attempt to graft you before receiving your unsub, and since this is not allowed, we'll prune him again with a bigger backoff.

[zah]

Please don't merge yet until we ship 1.5.5

[Menduist]

Unstable will not be merged into master, and thus get bumped in nimbus, for the next few days/weeks, so 1.5.5 won't be affected anyway

Or you have reservation about this PR in general?

[zah]

We haven't yet decided on the branching point from unstable that will go into the release. We avoid making commits to the testing and stable branches in preparation for releases.