Add new DomainType for application usage.

[ralexstokes]

This PR adds a new DomainType for use by applications adjacent to the core protocol so they can leverage the signing machinery developed here when working with consensus types.

This PR only adds a single domain and expects applications using the domain type to mix in further unique tags for domain separation; otherwise they risk signature collisions.

def compute_domain_for_application(application_domain: SomeBytesType):
    protocol_domain = compute_domain(DOMAIN_APPLICATION) # adjust other args as required...
    return Domain(hash(application_domain + protocol_domain))

An example use case is the work-in-progress Builder API where a validator may want to outsource block construction to a network of builders external to their local execution client. See ethereum/execution-apis#209 for details.

[ralexstokes]

@djrtwo, @lightclient, @metachris

[djrtwo]

I'm okay with either this or setting a high bit to give applications a large space to avoid any accidental signing.
Problem with that is that then you need to have applications be abl to register these somewhere to avoid picking the same values...

[ralexstokes]

closing in lieu of #2884.