Support for other discoverable metadata

Give tap developers the ability to declare fields unsupported and fields selected by default, according to the singer spec around metadata.

Relates to #91 (closed)

The documentation should also be updated:

• docs/implementation/catalog_metatadata.md: https://sdk.meltano.com/en/latest/implementation/catalog_metadata.html

Limited to:

• "selected-by-default": ...
• "inclusion": "unsupported"