Breaking a2 into separate crates

[pimeys]

To make it more modular, we could brake a2 into smaller crates:

• [ ] a2-types: request/response builders and errors
• [ ] a2-client: futures-based client using hyper/h2 behind the scenes
• [ ] a2-actix: an alternative, actix based client
• [x] hyper-alpn: the alpn connector
• [ ] jwt-signer: generates a JWT token and caches it for a given amount of time

[pimeys]

https://github.com/pimeys/hyper-alpn

[pimeys]

As separate as needed.