Add available space memory to /v3/spaces/:guid/usage_summary response

After redesigning one of the pages in SAP BTP Cockpit, we would like to include a key metric - the available memory of a space.
Currently, the endpoint /v3/spaces/:guid/usage_summary provides information about used memory, but to calculate the available memory we need additional logic on the client side:

• Detect whether the space uses a space quota or defaults to the org quota.
• Apply different calculations based on this.
• Perform extra API calls to retrieve quota details.

Since the page already aggregates multiple metrics, every additional call affects the performance.

Our proposal is to enhance the response of /v3/spaces/:guid/usage_summary to include a new field available_memory_in_mb, which should represent the remaining memory capacity available for allocations within the space, respecting both space and org quotas:

• If the space has a space quota assigned: available_memory = space_quota_memory_limit - used_memory
• If the space has no space quota: available_memory = org_quota_memory_limit - used_memory
• If the space quota is greater than the org’s remaining available memory, the available memory must be capped by the org: available_memory = min(space_quota_memory_limit, org_available_memory) - used_memory

Start by locating the implementation of the /v3/spaces/:guid/usage_summary endpoint and the existing quota and used-memory calculations. Verify the response for spaces with a space quota, spaces using the org quota, and cases where the space quota exceeds remaining org capacity; done means available_memory_in_mb follows the stated capped calculation.