Revise the OpenAPI validator / linter

[jparise]

• Tweak the reported names to match the existing conventions
• Use --version to query the version string (and trim() it)
• Request --verbose JSON output to also report the rule names
• Use the rule name as the linter message's "code"
• Document that we now support v0.34.1 and later

[steverice]

The --verbose option was only added in a recent version of the openapi-validator. Before that the equivalent flag was --print_rule_names.

We should have some nod to backwards compatibility. I think any of these would be acceptable:

• check the validator version and report an error if it is < 0.33
• check the validator version and add the appropriate flag accordingly
• mention in the readme that version >= 0.33 is required

[jparise]

I landed on just adding a note to the README.md about the version requirement. There unfortunately isn't a way for an individual linter to assert a default version requirement, and reimplementing the version check locally feels like too much work for this situation.

I also made the parser handle missing rules, and because the tool doesn't error for unknown command line options, we don't need to add it conditionally.