Fix OAuth client to match the documentation and current api

[guidozanon]

Remove AccessToken code and accept client_id & client_secret

[romerosilva-meli]

Hi @guidozanon, how are you?

Thank you for your contribution. Unfortunately both methods to create oauth credentials (with access token and client id/client secret) are valid. In order to accept your contribution, I'll ask you to refactor your code and rewrite it as an overloaded method, keeping the original as is.

[guidozanon]

Hey @romerosilva-meli, thanks for the reply. How is the current implementation match this documentation: https://www.mercadopago.com.ar/developers/es/reference/oauth/_oauth_token/post?

The current implementation does not receive the clientid/clientsecret to generate a token. Do you have an example of a working call with the current implementation? Maybe we are doing something wrong.

[romerosilva-meli]

You're right, the current documentation does not reflect that implementation. It has to be updated.

You don't need to worry about the current implementation, you can keep it as is in the SDK, you can just copy your implementation into a new, overloaded method, like this:

// original method
public Task<OAuthCredential> CreateOAuthCredentialAsync(
            string authorizationCode,
            string redirectUri,
            RequestOptions requestOptions = null,
            CancellationToken cancellationToken = default)
        {
          ...
        }

// overloaded method with your implementation
public Task<OAuthCredential> CreateOAuthCredentialAsync(
            string authorizationCode,
            string clientId,
            string clientSecret,
            string redirectUri,
            RequestOptions requestOptions = null,
            CancellationToken cancellationToken = default)
        {
          ...
        }

[romerosilva-meli]

Changes present in this branch were added to #169