[JS] (#7913) Adding support for multi file upload.

[tray2100]

To fix #7913

Adding support for multi file upload. Also adding the option for individual CodeGens to specify a collectionFormat for CodegenParameters

• updated samples as well

See the issue for details on the implementation.

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CC: @CodeNinjai @frol @cliffano @wing328

[wing328]

@tray2100 thanks for the PR. Can you please resolve the merge conflicts when you've time?

[wing328]

cc @OpenAPITools/generator-core-team as the PR impacts the Default Codegen class.

[tray2100]

@wing328 You're welcome for the PR! I've resolved the conflicts.