Headers-only support in graphsync fetcher

[ZenGround0]

Description

A recent spec proposal reiterates the need to separate header fetching from full block fetching. A first step to achieve this is to add headers-only fetching support to the graphsync fetcher.

Acceptance criteria

The graphsync fetcher is extended to include functions for fetching headers only.

Risks + pitfalls

• Selector foo is hard
• tests are already big and unwieldy, let's try to make tests cleaner with this PR

Where to begin

Probably the vast majority of fetcher logic can be abstracted into a common method figure out a nice shared abstraction.