Metadata endpoint support in engine

[dmcgowan]

Update the engine registry client to use the new metadata endpoint specification. This allows for simplifying the engine-side logic for configuring the registry endpoints.

See #80 for specification

[stevvooe]

@dmcgowan Are we still planning to do this?

[dmcgowan]

Yes, I plan on adding the specification to the namespace discovery conversation. I am holding up on further development until we have a more complete namespace specification.

[stevvooe]

We are probably going a very different direction here.