Introduce Github identity provider implementation in API Gateway - with a workflow that works

[afsalthaj]

This should be dead simple and can be done in the following

*Let's say user specified the security scheme and registered with golem under a project. For this task, let's assume this is done already and security scheme is already available for a particular in GitHub. In OSS, this security scheme can be a table not owned by any domain such aa project. This assumption implies, we will have a single security scheme stored in the table.

• From the perspective of API Gateway, when a user hits a configured secured endpoint, it understands that it is a secured endpoint probably because its gateway binding is configured with security scheme middleware. Before it goes on and evaluate the rib (hitting the worker), it checks if the request has an active session or access token, and if so go on with serving the request. If not the API Gateway redirects the user to GitHub for authentication, and in the request URL, it includes a state parameter that holds information about the original API endpoint they wanted to access.
• User authenticate themselves with the GitHub. and GitHub redirects to a dedicated API Gateway endpoint with the state parameter included in the query string.
• API Gateway now needs to exchange authorization code for an access token, and once it receives, it stores this information into the session and the redirects back to the original URL, and this time, the sessions will be active, and it goes on and execute the rib script with the details in the session passed along with the other inputs to rib.

This basic workflow is the first step to implementing worker authentication