[FEATURE] Switch code generation to point to versioned OpenAPI specification

[Xtansia]

Is your feature request related to a problem?

Now that opensearch-api-specification publishes a versioned specification we should migrate the code generation to use it rather than a local copy.

As the code-gen and spec is still in high flux, it may be valuable to keep the current approach until the code-gen is stable, i.e. the current incremental stages are complete and we no longer filtering which operations to generate.