Add a WithReifier method to allow easier derivation of non-unixfs pathing

[willscott]

dependency injection in IPFS means we can only get a single non-parameterized injection of a fetcher.Factory or Resolver.

We provide a method for downstream interface based derivation of what sort of reification semantics are desired.

[aschmahmann]

dependency injection in IPFS means we can only get a single non-parameterized injection of a fetcher.Factory or Resolver.

Without any judgement on this PR in particular, you can have multiple instances of the same type with go-ipfs's fx dependency injection you just need to do one of:

1. Use a type alias (https://github.com/ipfs/go-ipfs/blob/0e73e235678e95e164b7e5e4f23d2b588bea7afd/core/node/libp2p/routing.go#L24)
2. Use Go tags (example: https://github.com/ipfs/go-ipfs/blob/0e73e235678e95e164b7e5e4f23d2b588bea7afd/core/core.go#L103)

---

That aside having helper functions like WithReifier or allowing the reifier to be passed into the constructor as an option seem potentially useful.