We've changed the config to be more consistent about "copies" and
"artifacts". This means that "copies" no longer accepts a string, but a
slice of strings. We should bump the config version and blubber version
to make it clear that we're not longer backwards compatible.

Change-Id: Ib68f623cbdcdf0aed74f4fdaec062835748e8eb9

Wrote an OpenAPI 3.0 spec for Blubberoid that provides `x-amples`
entries compatible with service-checker.

The written spec includes basic schema for Blubber config objects that
may be later factored out for use in validation.

Note that OpenAPI 3.0 supports only the v4 draft of the JSON Schema
standard, so some parts of the configuration could not be fully
described. Specifically, v4 does not include the `patternProperties`
definition introduced in the JSON Schema v6 draft that would allow us to
describe `variants` and `runs.environment` and everything beneath.

Blubberoid was refactored slightly to incorporate the new spec as well
as assume JSON as the canonical and default configuration format. It was
also refactored to include a versioned namespace ("v1") after the server
endpoint.

Bug: T205920
Change-Id: I28a341aa503b8920d802715660d4c4d62be45475