feat: support patternParameters and typed additionalParameters

[paragon]

The goal of this PR is to support the ability to specify either:

additionalProperties:
    type: string

...or...

patternProperties:
    "regex_expr":
        type: string

I've created a test case here that utilizes both additionalProperties and patternProperties and succeeds in creating the associated codegen. Most notably, the generated schema file (internal/integration/test_additionalpropertiespatternproperties/oas_schemas_gen.go) seems to be look good which (at least, to me) indicates things are behaving as expected.

I'm hoping this is the correct approach here. If not, I'd be happy to start a discussion on best approaches to facilitate supporting additionalProperties and patternProperties.

As an aside: it seems like based on prior discussions that there has been hesitation in supporting additionalProperties: true. As such, this PR does not aim to support it.