Allow injection of custom Elasticsearch/OpenSearch mappings via configuration

[bountx]

Currently, the application uses a hardcoded default mapping configuration (ES_ITEMS_MAPPINGS). This creates two main issues:

• Rigidity: Supporting specific STAC extensions (SAR, Cube) or custom fields requires modifying source code or relying on dynamic templates (which may guess types incorrectly).
• Performance: Dynamic mapping is enabled by default, meaning Elasticsearch indexes every field in the ingested JSON. For large datasets with extensive metadata that isn't queried, this creates unnecessary overhead.

Proposed Solution:
We need a configuration-based approach to extend mappings and control indexing behavior without code changes.

1. Custom Mappings Injection

• Introduce STAC_FASTAPI_ES_CUSTOM_MAPPINGS (env var).
• Accepts a JSON string representing a properties dictionary.
• Behavior: This dictionary will be merged into the default properties.
• Override Policy: Custom mappings will overwrite defaults if keys collide. This allows users to refine types but ensures core fields (geometry, datetime) remain unless explicitly redefined (which should be done with caution).

2. Dynamic Mapping Control

• Introduce STAC_FASTAPI_ES_DYNAMIC_MAPPING (env var).
• Default: true (Maintains backward compatibility).
• Option: false (or strict).
• Benefit: Setting this to false allows users to define only the fields they want to query via the custom mappings variable, preventing the database from creating mappings for unused data and improving ingestion performance.