Allow custom configuration of transports

[hannahhoward]

Goals

Support the end goal of loading/storing storage/retrieval deals from custom block stores

Implementation

This is a bit fun/funky, but there are two basic adds:

1. A general purpose system for hooking in on a given voucher type to allow configuration of the the transport medium. (This is a bit of an abstraction breaker, cause the configurer receives a generic transport interface, that they cast to the transport medium they want to configure -- though I think this is ok, because once we have multiple transport options, the configurer may want to take different actions for different transports -- it feels silly to try to build a generic interface for configuring transports -- the things you do with each will be different)

Anyway this is done with by registering a transport configurer which has this signature:

type TransportConfigurer func(datatransfer.ChannelID, datatransfer.Voucher, datatransfer.transport)

(oy Configurer... that's a word -- maybe there is something better?)

2. A method for customizing the loader/storer for a given request on the graphsync transport medium. This uses graphsync's alternate persistence options under the hood.

You can see how this all comes together in the TestRoundTrip in integration test.

I also did some rearranging of files to avoid circular imports and break up the types file.

[hannahhoward]

arg... I really didn't mean for this to seem like such a large PR -- the rearranging of files to avoid the circular imports really grew quick. I'd focus on graphsync transport changes, and the implementation of the registry of configuration functions

[ingar]

Looks good, types cleanup FTW. One question about the test configuration; I didn't look too closely.

[ingar]

should this be data.customTargetStore?