Quick and harmless fix to help with alternate OpenSearch schemas (#553)

**Related Issue(s):**

- Partly helps with
#544
(I did not want to create a new ticket which could be redundant with
this one)

**Description:**

Have opensearch datetime, geometry and collections fields defined as
constant string so to be easily overwritten when the OpenSearch mapping
requires it, as shown in example
https://github.com/NASA-PDS/registry-stac-api/blob/464ce37952367fa845b325e36263b04a1fd89965/src/pds/registry/stac/database_logic.py#L28

**PR Checklist:**

- [x] Code is formatted and linted (run `pre-commit run --all-files`)
- [x] Tests pass (run `make test`)
- [x] Documentation has been updated to reflect changes, if applicable
- [x] Changes are added to the changelog

---------

Co-authored-by: thomas loubrieu <thomas.loubrieu@jpl.nasa.gov>
Co-authored-by: Jonathan Healy <jonathan.d.healy@gmail.com>

Author: 3 people

CHANGELOG.md

@@ -17,8 +17,11 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Added GET `/catalogs/{catalog_id}/children` endpoint implementing the STAC Children extension for efficient hierarchical catalog browsing. Supports type filtering (?type=Catalog|Collection), pagination, and returns numberReturned/numberMatched counts at the top level. [#558](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/558)
 - Implemented context-aware dynamic linking: catalogs use dynamic `rel="children"` links pointing to the `/catalogs/{id}/children` endpoint, and collections have context-dependent `rel="parent"` links (pointing to catalog when accessed via `/catalogs/{id}/collections/{id}`, or root when accessed via `/collections/{id}`). Catalog links are only injected in catalog context. This eliminates race conditions and ensures consistency with parent_ids relationships. [#559](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/559)
 
+
 ### Changed
 
+- Have opensearch datetime, geometry and collections fields defined as constant strings [#553](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/553)
+
 ### Fixed
 
 - Fix unawaited coroutine in `stac_fastapi.core.core`. [#551](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/551)