feat/return-cookies

[frectonz]

This PR introduces support for the returnCookies field, which behaves similarly to the returnHeaders field. When enabled, the returnCookies field will parse all the cookies set by an endpoint and return them.

This feature is particularly useful for individuals using better auth as a server-only library. It allows them to avoid manually parsing the set cookie headers when they want to set them themselves via their preferred web framework.

better-auth/better-auth#3851

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[frectonz]

Couldn't sign the CLA, got this error.