Add support for RBAC API get role endpoint

[danielkee]

Resolves ticket: CISC-3191

Summary of changes:

• Added support for the PE RBAC API endpoint GET /role/:id
• Added an accompanying unit test and integration test

Integration test output:

% go run cmd/test/main.go epicyclical-pro.delivery.puppetlabs.net AL40f61YgFIhtIT_n1XBeWTbclTilG2r8NTSFX7ndHxP
Connecting to: epicyclical-pro.delivery.puppetlabs.net

* Testing RBAC API client function CreateRole...

Create role "Testing 1680519311359000000" was successful, location: /rbac-api/v1/roles/12
Create role "Testing 1680519311359000000" failed as expected because role already exists

* Testing RBAC API client function GetRole...

Get role was successful, as the role "Testing 1680519311359000000" was returned in response

* Testing RBAC API client function GetRoles...

Get roles was successful, as role "Testing 1680519311359000000" was present in response