Support '__all__' in no_db_index to exclude all fields from indexing in the Historical Model

[rajarehanahmed]

Description

Adds support for using __all__ in the no_db_index to exclude all fields from being indexed in the historical model.

Related Issue

Closes #1491

Motivation and Context

Although fields can be excluded from indexing individually via no_db_index, there's currently no simple way to exclude all fields at once. This adds a cleaner approach using '__all__'.

How Has This Been Tested?

Added a unit test to confirm that setting no_db_index = '__all__' results in no indexed fields in the historical model.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the pre-commit run command to format and lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.