feat: add extra admin tools, update unit test and readme

[pkalsi97]

Issue #159

Summary

This PR adds 7 new admin tools:

List roles

1. list_all_roles – list all Grafana roles with optional delegatable filter
2. get_role_details – fetch detailed info for a role by UID

List assignments of roles

3. get_role_assignments – list all users/teams/service accounts assigned to a role
4. list_user_roles – list all roles assigned to specific users
5. list_team_roles – list all roles assigned to specific teams

Debug role assignments

6. get_resource_permissions – list all permissions defined on a resource
7. get_resource_description – describe available permissions for a resource type

Also Updates admin_test.go by adding new tools and README.md by adding required information regarding the tools.

Note for reviewer

Not sure how to set up integration test for these tools as these tools only work in Grafana Cloud and Enterprise, please give suggestions incase Integration test is necessary.

@ioanarm, please review this when you see fit, looking forward to valuable feedback.