Add Provider Schema support with new `schema_ref` config option

[austinvalle]

This PR includes:

• New provider.schema_ref property which can be any valid/existing JSON schema reference in an OpenAPI spec.
• All of the associated mapping functions/tests for the oas package (including validators most recently added)
• Addition to edgecase test for the provider schema scenario
• Upgraded libopenapi to v0.9.2
  • This included a breaking change to Schema.Maximum and Schema.Minimum fixed in 8fd0d5b437aee48930cba7e4d54245acc724205b
  • Go conversion rules state this will just chop off the decimal place in the scenario of it being a float, so this should be okay?
• Provider docs and examples for generator configs

I don't believe this PR needs to make any changes related to #39, so will merge after.

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