[DEV-12966] Add pagination to spending_by_subaward_grouped endpoint

[aguest-kc]

Description:

Updated the /api/v2/search/spending_by_subaward_grouped/ endpoint to correctly paginate results and to sort results within Elasticsearch instead of in Python.

Technical Details:

I added an ad-hoc agg key that combines the Award ID and Award unique key fields together to prevent creating sub-bucket aggregations which are hard to sort on.

Requirements for PR Merge:

1. Unit & integration tests updated
2. Data validation completed (examples listed below)
   1. Does this work well with the current frontend? Or is the frontend aware of a needed change?
   2. Is performance impacted in the changes (e.g., API, pipeline, downloads, etc.)?
   3. Is the expected data returned with the expected format?
3. Jira Ticket(s)
   1. DEV-12966

Explain N/A in above checklist:

2. API documentation updated (examples listed below)
   No API contracts changes are necessary.

3. Appropriate Operations ticket(s) created
   No operations tickets are required for this change.