fix: handle paginated response

[boonware]

Issue

spinnaker/spinnaker#6768

Description

• Current code does not handle a paginated response from GitHub API. If the user is a member of more the 30 GitHub orgs (the default page size), and the target org is not contained in the first page, then the org membership check will fail.
• This change handles paginated response by parsing out the link to the next page in the Link header, for example: '<https://github.com/api/v3/users/1234/orgs?page=2>; rel="next", <https://github.com/api/v3/users/1234/orgs?page=3>; rel="last"'.
• Unit test suite is included. In order to make the code testable, the private modifier was removed from the instance variable restTemplate to allow dependency mocking. A better design would inject this dependency, but this is outside the scope of the current issue.

[boonware]

@dbyron-sf Could I please get a build approved? Thank you.

[mattgogerly]

Hi @boonware, could you update your commit message to fix(github) and I can approve the workflow.