[ALL-GENS] - Feature - Remove vendor extensions from operations and schemas; Add vendor extensions to operations and schemas without touching open api spec

[Picazsoo]

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• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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Summary by cubic

Enable sideloading of OpenAPI vendor extensions for models and operations, so generators can add or remove extensions (e.g., x-kotlin-implements) via configuration without editing the source spec. This helps align outputs across generators and simplifies customization.

• New Features

  • New config keys: modelVendorExtensionAdd/remove and operationVendorExtensionAdd/remove (in additionalProperties).
  • Models: support class-level ("class") and field-level ("fields") add/remove; added values are merged with existing ones.
  • Operations: support method-level ("method") and parameter-level ("parameters"/"params") add/remove; added values are merged.

• Refactors

  • Consolidated Kotlin Spring server tests and removed a duplicate test file.
  • Added tests for adding/removing x-kotlin-implements and a new spec (petstore-without-x-kotlin-implements.yaml).
  • Kept gradle wrapper generation tests in the updated suite.

^{Written for commit 37e142e. Summary will update on new commits.}