Convert OpenAPI 2.0 to 3.0

[jfinkhaeuser]

Expected Behaviour

A new command should convert OpenAPI 2.0 specs to 3.0, as well as possible. In code, this should be functionality to run on a parser's spec, so we can use it on the BaseParser prior to resolving references.

Tests would require that converted specs validate as 3.0.

[jfinkhaeuser]

This looks done. It's simpler than expected, because I'm just using a web API - let's see how far we get with that.