Extract code_challenge_methods_supported from discovery JSON

[mitar]

By at least extracting code_challenge_methods_supported we could know if the provider supports PKCE. Then one can use oauth2.S256ChallengeOption to add challenge to the AuthCodeURL.

[mitar]

Oh, I can use provider.Claims to extract that:

var claims struct {
    CodeChallengeMethodsSupported []string `json:"code_challenge_methods_supported"`
}
err := provider.Claims(&claims)
if err != nil {
    panic(err)
}
supportsPKCE := slices.Contains(claims.CodeChallengeMethodsSupported, "S256")