Possibility to Authorize as GET request without navigating to browser

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When calling authorize, an AuthorizationRequest can be created with all information that is on hand, and values for nonce, codeVerifier/codeChallenge and state are created within the package via method channels when calling authorize, after which the browser is opened with that input.

However, we require a GET request for the authorization first step and need to handle the response manually since we navigate to a separate authorization app using elements of the response.
Currently, we therefore manually create the required request, filling values for nonce, codeVerifier/codeChallenge and state, and perform a GET with an url compiled from that request. But we would really appreciate it if this can be done via your package.

Start at the package's authorize entry point and AuthorizationRequest handling, then trace the method-channel creation of nonce, codeVerifier/codeChallenge, and state before browser launch. Define an API that exposes the authorization GET request and its response without navigation, covering both Android and iOS behavior. Done means callers can perform the first request and handle its response manually.

Written by the indexing model from the issue text.