feat: add experimental optimistic provide

[guseggert]

This adds the ability to enable "optimistic provide" to the default
DHT client, which enables faster provides and reprovides.

For more information about optimistic provide, see:

https://protocollabs.notion.site/Optimistic-Provide-2c79745820fa45649d48de038516b814

Note that this feature only works when using non-custom router
types. This does not include the ability to enable optimistic provide
on custom routers for now, to minimize the footprint of this
experimental feature. We intend on continuing to test this and improve
the UX, which may or may not involve adding configuration for it to
custom routers. We also plan on refactoring/redesigning custom routers
more broadly so I don't want this to add more effort for maintainers
and confusion for users.

[guseggert]

This needs docs in docs/experimental-features.md and a changelog entry

[BigLep]

Aprovig because:

1. Logically the changes made sense to me.
2. This is an experimental feature
3. The docs looked fine
4. I know others have looked at this too

[BigLep]

A few other thoughts:

1. I assume you'll add to the changelog in docs: preparing 0.20 changelog for release #9799
2. Are there other issues or PR's we should link to (e.g., Optimistic Provide libp2p/go-libp2p-kad-dht#783 )
3. Also, in the experimental feature docs maybe get links to more info like https://protocollabs.notion.site/Optimistic-Provide-2c79745820fa45649d48de038516b814 and DHT Optimistic Provide libp2p/notes#25