Set Repository size limit at the group/project level using the API
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### Description
Repository size limit can currently only be set globally via the [Settings (application) API](https://docs.gitlab.com/ee/api/settings.html#change-application-settings). However, there's no way to configure this limit at the group level or for individual repositories.
### Proposal
Provide an API for setting repository size limit on a per group/repository basis.
### Links / references
- Complements #559
- EE customer has requested this feature: [ZenDesk 73839](https://gitlab.zendesk.com/agent/tickets/73839)
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