Expose adapters_by_mimetype config in HTTP API somehow

[danielballan]

This is spun off from #661. The goal is to enable client registration (tiled register ...) to use same adapters that server has.

Considerations:

• Different catalogs in the same server may have different adapter_by_mimetype. That is, this is per-catalog, not per-server, so it's not immediately clear where/how to best expose it.
• Do we want to expose anything about the versions? This could raise security concerns, but worth considering if and to what extent we want to help clients realize that they may have an out of sync version.

[padraic-shafer]

If the call to register automatically selects the lowest matching tree, then it should use those adapters. Does the client need access to those same adapters, or can the server handle everything at that point?

(Or where does the current WIP break down in this regard?)