Embedded and in-memory lookups should be common across http modules

[istreeter]

We maintain a couple of different implementations of a RegistryLookup, e.g. one based on java.net.http.HttpClient and one based on http4s.

The different implementations mainly differ in how they do http lookups. But they also need to provide implementations for lookups in non-http registries (embedded and in-memory). Over time, the implementations for the latter registry types have drifted apart. For example, this RegistryLookup can "list" schemas in an embedded registry (we added the feature in #212) but the http4s RegistryLookup returns NotFound for the equivalent lookup.

Another example... the http4s registry lookup recovers from all exceptions on a http lookup whereas the java.net.http.HttpClient registry lookup does not catch errors when listing schemas.

We can refactor the code so that a lot more is shared between the different implementations.