[Fixes #11902] Add better support for ISO19115-3:2018 metadata format

[cmotadev]

This PR is a proposal to allow GeoNode to set ISO19115-3 (2018) as default metadata format, instead of ISO 19139.

Features:

• Introduced DefaultMetadataFormatMixin to manage default metadata formats.
• Updated Catalogue and CatalogueBackend classes to utilize the new mixin for default format and schema.
• Modified pycsw_http and pycsw_local backends to dynamically set supported formats based on METADATA_FORMATS.
• Implemented conditional import of MD_Metadata based on the selected metadata format.
• Created full_metadata_iso19115-3.xml template for ISO19115-3:2018 metadata.
• Updated settings.py to allow configuration of the default metadata format and template.
• Adjusted tests to accommodate changes in metadata format handling.

Checklist

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[gemini-code-assist]

Code Review

This pull request introduces support for the ISO 19115-3:2018 metadata standard by adding a new XML template, a metadata format mixin, and dynamic backend configuration. Review feedback identified several high-severity issues in the new template, including undefined variables, inverted logic for contact details, syntax errors in template tags, and potential crashes from missing null checks. Suggestions were also made to improve configuration validation in settings.py and to use more specific exception types in the local backend.