JDK11 HTTP client used for OAuth2 authorization endpoint should support OAuth2 client authentication

[ldclakmal]

Description
The JDK11 HTTP client implementation which is used in oauth2 module in order to send requests to 'authorization server', should support OAuth2 client authentication mechanisms. This support is provided for Ballerina 1.2.x versions but it was removed since the client implementation is moved from Ballerina to JDK11 HTTP client.

[ldclakmal]

The client of oauth2 module can only be provided OAuth2 grant types supported by Ballerina. Because with the introduction of other auth schemes, there will be cyclic dependency issues. Same applies for jwt module client as well.

Sample implementation for oauth2 module:

public type ClientConfiguration record {|
    HttpVersion httpVersion = HTTP_1_1;
    map<string> customHeaders?;
    string customPayload?;
    ClientAuth auth?;
    SecureSocket secureSocket?;
|};

public type ClientAuth ClientCredentialsGrantConfig|PasswordGrantConfig|DirectTokenConfig;

public class ListenerOAuth2Provider {

    public isolated function init(IntrospectionConfig introspectionConfig) {
        // ...
        ClientAuth? auth = introspectionConfig.clientConfig?.auth;
        if (auth is ClientAuth) {
            ClientOAuth2Provider clientOAuth2Provider = new(auth);
            string|Error accessToken = clientOAuth2Provider.generateToken();
            if (accessToken is string) {
                map<string>? customHeadersMap = introspectionConfig.clientConfig?.customHeaders;
                if (customHeadersMap is map<string>) {
                    customHeadersMap["Authorization"] = "Bearer " + accessToken;
                } else {
                    introspectionConfig.clientConfig.customHeaders = { "Authorization" : "Bearer " + accessToken };
                }
            }
        }
    }
// ...
}