internalauth: take both ends of the internal protocol from ecore

The guard on /internal/repos and the header cmd/dolt-git-hook minted for it
were two hand-written halves of one protocol in two packages that shared no
type, no constant and no test. Both are now sr-ht-ecore/internalauth: Guard on
the receiving end, AuthorizationAs on the calling one, over one Auth struct.

The guard also pins the caller, which the old copy did not: core-go only asks
that a token name some client and node, and on an endpoint that provisions a
database for an arbitrary user that means any holder of the network key will
do. The pinned pair lives in core so the mint and the pin cannot drift apart.

The hook test now runs internalauth.Identify — the real receiving end — over
the header the hook produced, so the two ends are checked against each other
rather than against a third copy of the decode.