Add GraphQL to read and update usage of instance runners in group

Description

Currently under Group -> Settings -> CI/CD we offer a way for our Admin users to change the following settings:

• A new field(s) in the Group GraphQL entity to return the current state
• A new mutation to update the field(s) in the group.

Implementation notes

The group has 3 states to represent the group's usage of instance runners.

• enabled: Group can use instance runners
• disabled_with_override: Users of sub-groups and projects can enable usage of instance runners
• disabled_and_unoverridable: Users of sub-groups and projects cannot enable usage of instance runners