Support JSON Schema propertyNames, dependentSchemas, if, then, and else on IOpenApiSchema #2878

Is your feature request related to a problem? Please describe.
A few JSON Schema 2020-12 keywords are still missing from IOpenApiSchema, which prevents representing and round-tripping those schemas through the strongly typed model.

Describe the solution you'd like
Add first-class support for the following JSON Schema 2020-12 keywords on IOpenApiSchema and the related schema implementations:

• propertyNames
• dependentSchemas
• if
• then
• else

This should include the interface, OpenApiSchema, OpenApiSchemaReference, constants, reader/deserializer support, and 3.1+/3.2 serialization behavior.

Describe alternatives you've considered
These keywords can currently only be preserved as unrecognized keywords, which does not provide a typed object model surface.

Additional context
unevaluatedItems was already handled in PR #2728, and the contains family was already handled in PR #2876, so this issue is limited to the remaining keywords above.