Custom Mappings and Custom Sortable Fields List

[YafetBF]

Sortable fields
A new approach is included regarding custom mappings. With custom mappings if we include new field as keyword, sorting based on that specific field is necessary. Sortable fields are already defined in the database_logic. However, the number of fields are limited to four only. Adding new field into sortable fields can be considered from the environmental variables after custom mappings are applied.
Example: - COLLECTIONS_SORTABLE_FIELDS=id,extent.temporal.interval,temporal,title

Custom Mappings from Config File
A custom mappings json template similar to the default template can be stored in config folder. Users can easily edit and apply it from the environmental variables. The sample config json is important to highlight general structure of the mappings json. This is traceable and can be used as master template.
Example: - STAC_CUSTOM_MAPPINGS_PATH=/app/stac_fastapi/config/config_mappings.json

Alternatives I've considered
The above two features are demonstrated in a pull request here #577

[YafetBF]

Custom Mappings is already integreted in the project. It is recomeded to use existing approach which is well documented on the Readme.

Regarding Sortable Fields Restriction, a new pull request is created #582
Instead of adding Sortable Fields every time mapping is customized, it is suggested to remove restrictions so that it will be handled by Elasticsearch/Opensearch backend. In the PR, the restrictions are removed.