OAuth2 PKCE extension

[ManHatos]

https://datatracker.ietf.org/doc/html/rfc7636
Looks like Discord's OAuth2 implementation partially supports the extension
Before I finish this PR is there a reason for it not being documented?

Closes #5254

[ManHatos]

Notes:

1. As per Support Custom Scheme Redirect URIs in OAuth2 #450 (comment) + Deep Linking with OAuth2 Not Working. #1296 (comment), clients with custom redirect URIs must use PKCE
2. As per section 4.1 of the spec, the code_verifier must be at least 43 chars long, Discord strictly enforces this
3. (?) the role_connections.write scope does not support PKCE (Cannot use role_connections.write scope with PKCE #5751 (comment))
4. refresh_token grants do not support PKCE OAuth2: Refreshing does not work with PKCE #5531
5. Only S256 for code_challenge_method is supported otherwise an invalid_request error is returned Unsupported "code_challenge_method"