OAuth2Request should access grant_type through data

[apvd]

The grant_type property of the OAuth2Request class should refer to the data collection (consistent with the other properties response_type, client_id, etc. and not through the base form. Without this change the following grant setting will not function:

TOKEN_ENDPOINT_HTTP_METHODS = ["GET"]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

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[codespearhead]

I'd argue that's by design, as stated here: #658 .

[apvd]

RFC frowns upon GET requests for token endpoint.
https://datatracker.ietf.org/doc/html/rfc6749#section-3.2