Add PKCE option to Oauth2Provider

[hazelnutcloud]

Closes #148

The x.com provider does not work out of the box because the x.com oauth2 does not work without PKCE. See the "Grant Types" section in https://developer.x.com/en/docs/authentication/oauth-2-0/authorization-code. This PR adds an option to the Oauth2Provider base function to include PKCE flow which fixes the x.com provider.

[changeset-bot]

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Latest commit: 0e6e4cb

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