BioThings Explorer Should be able to handle APIs which couldn't be called

[kevinxin90]

BioThings Explorer has the capability to display input/output information for each API endpoint.
We should categorize APIs into two separate categories:

1. APIs which has metadata ready and can be called
2. APIs which has metadata ready but can not be called, due to
   a) They doesn't return JSON format, e.g. SQL, SPARQL query endpoints
   b) They accepts multiple parameters (currently we handle one parameter only)
   c) The return format cannot be JSON-LD labeled, e.g. pharos

Those API still should be discoverable using BioThings Explorer, but when making actual API call chains, they should be excluded.

TODO:
Each API endpoints in the BioThings Explorer registry should have a field called tag, which categorize their functionality.