feat(facets): Unify __unknown__ and __unknown-facet__ in a constant

[annacv]

Pull request template

Create UNKNOWN_FACET_KEY constant to identify filters without facetId (RawFilter).

BREAKING CHANGES:

• The '__unknown-facet__' value for a facetId is deprecated: it has been replaced by the UNKNOWN_FACET_KEY constant, whose value is '__unknown__'.

Motivation and context

Right now we’re identifying unknown facets giving them either __unknown__ or __unknown-facet__ as id.
We should unify this and use a constant instead of a plain string.

• Dependencies. If any, specify:
• Open issue. If applicable, link: EMP-2674

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that causes existing functionality to not work as expected)
• Change requires a documentation update

What is the destination branch of this PR?

• Main
• Other. Specify:

How has this been tested?

Tests performed according to testing guidelines:

Checklist:

• My code follows the style guidelines of this project.
• I have performed a self-review on my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

[CachedaCodes]

This is the file for the constants that we're injecting in several places of the app, we're not injecting the new constant anywhere so it shouldn't be here.
A better place would be in the facets module. See the history queries module for an example.