Bug 1973636 - Add isNewUntil Search Definition to Rust Engine Selector.

[mandysGit]

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[Standard8]

Thank you, this is looking good, but there's a couple of extra bits to add:

• Please add an entry to CHANGELOG.md. Now that we are being used by multiple apps, we should be adding entries there.
• Please can we extend test_from_configuration_details_merges_variants and test_from_configuration_details_merges_sub_variants to test that is_new_until gets properly merged.

[mandysGit]

Addressed Review comments. Oops forget to push my changes. They're in the push below this comment.

After this lands, when may I land my other patch: https://phabricator.services.mozilla.com/D253104 ?
Would I need to wait until this code gets vendored into Mozilla Central first?

[Standard8]

Thank you, this looks good. For landing, it'll need to be vendored across.

I'll check to see if anyone is planning to do a vendor soon.