Add "extensions" field to contact()

[jonthegeek]

It is very common for the contact field to have extra bits, and sometimes only extra bits. Allow for that.

Example: https://api.apis.guru/v2/specs/fec.gov/1.0/openapi.yaml

Rather than dumping extra fields, lets give them a place to live.
Add a list-based extensions field to contact() (and, eventually, everything, but this ticket is only for contact()). Pull the "x-" off of those extra fields, then add them to that list.