feat: add exportToJSON

[wemeetagain]

Matches exportToProtobuf

[achingbrain]

I think we should really remove these createFromX and exportToX methods as from what I can see they don't get used anywhere outside of tests - it's pretty cheap to create Ed25519 keys as they are just 32 random bytes and we'll then be able to drop the protobuf deps from the peer-id-factory module.

Where are you using these methods?

[wemeetagain]

I was using to/from JSON to persist our network identity in lodestar. Not a huge thing if we decide to remove that here.

[BigLep]

Closing because of consensus that this functionality shouldn't be part of the library.