This patch adds an 'AuthenticatorResponse' interface, representing the
generic attributes of responses from authenticators. It then redefines
'ScopedCredentialInfo' and 'AuthenticatorAssertion' to derive from this
interface, and renames them to 'AuthenticatorAttestionResponse' and
'AuthenticatorAssertionResponse' respectively.

These new interfaces are a drop-in replacement for the old interfaces,
no normative changes are intended in this patch, other than the
renaming.

Passing a single dictionary parameter into `getAssertion()` provides
for greater forward compatibility, as new data can be flexibly added
to the method invocation without restructuring the existing
structure. It also helps developers understand what they're passing
in. This is less important for `getAssertion()` than it is for
`makeCredential()`, obviously, but aligning both in a similar
structure seems like a good change to make.

Passing a single dictionary parameter into `makrCredential()` provides
for greater forward compatibility, as new data can be flexibly added
to the method invocation without restructuring the existing
structure. It also helps developers understand what they're passing
in, as each parameter will be labeled.

This patch restructures the data passed into `makeCredential()`
substantially, moving from four parameters to a single dictionary,
and merging some existing types into a simpler structure. Most of
it is straightforward; the only bit I know will be controversial is
dropping `RelyingPartyUserInfo` in favor of two instances of a
simpler `ScopedCredentialEntity` object: one for the RP, one for the
user.

Let's chat about how (un)reasonable this approach might be.