Support relative date ranges in search

[chouinar]

Summary

This requires two sets of changes:

• Our API schema needs to allow relative date ranges (negative/positive integers representing days)
• We need to be able to pass in relative date ranges to the search index

I'd recommend first reading the docs on date ranges: https://opensearch.org/docs/latest/query-dsl/term/range/ and manually determining what the query needs to be to do this.

For the API schema, we currently support:

    "post_date": {
      "end_date": "2024-02-01",
      "start_date": "2024-01-01"
    }

We should allow:

    "post_date": {
      "end_date_relative": -15,
      "start_date_relative": 15
    }

These should be mutually exclusive, which may require some more complex rules on the schema than what we previously have done.

Acceptance criteria

• API supports relative date range (specific range still allowed/supported)
• Search client updated to support relative date range
• Tests added to validate this behavior