pass PKCE fields to AuthorizationView form

[skarzi]

Hi 👋

first of all thanks for developing and maintaining a great project!

Description of the Change

Pass code_challenge and code_challenge_method from query string to AuthorizationView form in get().
Without this, it was impossible to use authorization code grant flow with GET, because code_challenge and code_challenge_method data were never passed to form, so they weren't in form.cleaned_data, which causes creating Grant with always empty code_challenge and code_challenge_method.

This issue was quite hard bug to discover because there are already few tests for authorization code flow pkce, however, they weren't checking form rendering in GET request, but only response.status_code, I have added asserts for these 2 values, please look at the changes in test_public_pkce_plain_authorize_get and test_public_pkce_S256_authorize_get tests in test_authorization_code.py.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[coveralls]

Pull Request Test Coverage Report for Build 1494

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 94.821%

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Totals
Change from base Build 1485:	0.02%
Covered Lines:	1300
Relevant Lines:	1371

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💛 - Coveralls

[MattBlack85]

looks good @skarzi , checked the tests locally and they're ok, thanks!