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)

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

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

• [ ] Yes
• [X] No

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• [X] You consent that the copyright of your pull request source code belongs to Authlib's author.

[codespearhead]

I'd argue that's by design, as stated here: https://github.com/lepture/authlib/issues/658 .

[apvd]

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