Refactor to remove AuthenticatedContext and pass client to query functions

[sophiehouser]

We're removing the AuthenticatedContext pattern from the SDK since it's unidiomatic in Go to extend the context, save it as an attribute and use it to store data across multiple requests. We were previously using the AuthenticatedContext to save the request token across requests. It meant we didn't need to get the token freshly on every request. Instead we now store the token in the client and have users pass the same client to every request.

This will be a breaking change and will change the function signature of SDK functions.

This PR also creates a CONTRIBUTING.md file.

Summary of changes

• Moves all the token generation logic to the client
• Condenses code into one client package
• Makes the GenerateToken function a method on Client
• Removes MockTokenParams because the same functionality could be achieved with a GetTokenParams object

fixes FRA-6031

[vigneshwerv]

I think we should probably bump the major version to 3 and release it, when you merge the PR?

Should we update the README to include a guide for migrating from 1.x,2.x to 3.x?