Add Options_Conditional and use it for enable_headless_rest_endpoints.

[herregroen]

Context

• Introduces new conditionals with dynamic arguments and adds the first, the Options_Conditional. Implementing it for the headless REST API feature toggle.

Summary

This PR can be summarized in the following changelog entry:

• Deprecates the Headless_Rest_Endpoints_Enabled_Conditional.

Relevant technical choices:

• The Headless_Rest_Endpoints_Enabled_Conditional is replaced by the dynamic Options_Conditional.
• The Options_Conditional does not implement an interface as PHP interfaces are not expressive enough to allow for a dynamic number of arguments without forcing splat arguments on every implementation.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• The "REST API: Head endpoint" feature toggle should functional identically.
• See Add new endpoint for getting the head by URL #14848 and Add yoast_head property to relevant core REST routes #14883 for the original implementation.

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• The headless REST endpoint output, see the original two PRs for more details.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.