Please add a common trait for token requests

[Natureshadow]

I am writing a wrapper function that takes some configuration, then builds a request object with exchanged credentials (by calling the respective exchange_* methods on BasicClient).

The returned object is of one of the types in https://docs.rs/oauth2/4.0.0/oauth2/struct.ClientCredentialsTokenRequest.html?search=tokenrequest – however, these objects do not share a common trait, so cannot be returned by the same function and used the same way by the caller.

Ass all the *TokenRequest objects have the same API, it would be helpful to have a common trait that can be used as a type.

Along these lines, something like this:

pub enum Credentials {
    ClientCredentials,
    PasswordCredentials(String, String),
    // ...
}

pub fn exchange_credentials(&self, c: Credentials) -> TokenRequestTrait {
    match c {
        Credentials::ClientCredentials -> self.exchange_client_credentials(),
        Credentials::PasswordCredentials(u, p) -> self.exchange_password_credentials(...),
        //...
    }
}

would probably be a good idea as well.