Set the Header in the metadata feed to `hyperdrive`

[RangerMauve]

Currently, it looks like there's no way to differentiate between a Hyperdrive, regular Hypertrie, or some sort of MountableHypertrie.

This could make things hard for dynamic logic based on the type of a hypercore in stuff like pinning services.

I propose modifying the setMetadata method in hypertrie and mountable-hypertrie to accept an additional type parameter which we could use to pass the type into.

I think this will also help for when we have additional datastructures built on top of hypertrie in the ecosystem.

Alternately, would it make sense to add a subtype to the header to specify the additional type that's building on top of the hypertrie?

Refs:

• Hypertrie.setMetadata
• Hypertrie header writing
• MountableHypertrie.setMetadata
• Hyperdrive setting metadata