Adds cookie auth support to ApiKeyAuth in dart2 templates

[tek08]

Issue #7344

This modifies api_client.mustache and api_key_auth.mustache to allow an option of cookie for location of apiKey. To test, you can create a security schema such as:

securitySchemes:
  ExampleCookieAuth:
    type: apiKey
    in: cookie
    name: session

This should send a Cookie header up to server with semi-colon separated auth keys/values e.g.:

Cookie: session=12345; otherAuthKey=abcdef

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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[tek08]

Whoops. Embarrassing 😂. . Fixed. Thank you for catching.