This pull request enhances the OpenAPI emitter to support generating output files in multiple formats (YAML and JSON) simultaneously, rather than just one. It introduces the ability to specify an array of file types and updates the output file naming logic to accommodate this new feature. Comprehensive tests have been added to verify the correct behavior for various scenarios.

Multi-format output support

• The file-type option in OpenAPI3EmitterOptions can now be a single value or an array, allowing emission of both YAML and JSON files in one run. The schema and documentation were updated accordingly. [1] [2] [3]

• The output file name logic now uses the {file-type} variable when multiple formats are specified, ensuring distinct filenames for each format. The default value and documentation for output-file were updated to reflect this. [1] [2]

Emitter logic updates

• The emitter now iterates over all specified file types, generating the appropriate output for each service and version. The logic for resolving output file names and serializing documents was updated to handle multiple formats. [1] [2]

• The resolved emitter options and related interfaces were updated to use fileTypes: FileType[] instead of a single fileType.

Testing

• New tests were added to verify multi-format output, custom output file naming, and correct handling for multiple services and versioned services. These tests ensure that both YAML and JSON files are emitted as expected in all scenarios.