feat: provider manager option

[alanshaw]

The PR adds an option to allow passing your own instance of provider manager when constructing a DHT.

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In Hydra, we have many heads (up to 200 currently) that have their own DHT instances and corresponding provider managers. They are all caching records and performing GC on a shared datastore.

I'd like to avoid multiple GC runs and out-of-sync caches. In this PR I seek to fix this by enabling the DHT to accept a provider manager as an option, allowing a single provider manager to be used for all hydra heads.

We've defined a ProviderStore interface that ProviderManager (already) implements. By using an interface we give ourselves room to pass a custom implementation (at a later date maybe...since we will encounter the problem again when using the postgres datastore to share between Hydras, in addition to Hydra heads).

⚠️ BREAKING CHANGE - it removes the dht.ProviderManager property in favor of a dht.ProviderStore() function.