API to control build minutes limits at the namespace and group level

Description

The shared minutes quota can be controlled through the UI. However, we need the ability to change this value through the API, so we can pilot this from customers.gitlab.com, and enable the right limits after a customer purchases or changes a plan.

Proposal

• Add ability to control the build minute limits through the API
• We should be able to control both this setting at the group level, as well as the user namespace level

Links / references

• Shared minutes quota documentation: https://docs.gitlab.com/ee/user/admin_area/settings/continuous_integration.html#shared-runners-build-minutes-quota