Our use case is to pass additional URL parameters as part of the password credentials grant. Currently we need to hook into the context, create a RoundTripper that attaches extra args to the request, and inject that into the call to PasswordCredentialsToken.

If it supported options similarly to AuthCodeURL, this would be trivial to do from the client point of view instead.

I propose modifying

func (c *Config) PasswordCredentialsToken(ctx context.Context, username, password string) (*Token, error)

into

func (c *Config) PasswordCredentialsToken(ctx context.Context, username, password string, opts ...AuthCodeOption) (*Token, error)

If the AuthCodeOption naming is confusing, I imagine we could have a separate PasswordCredentialsOption type, but I'm not sure it's worth it.