Bug 2050097 - Add AI engine classification

[mandysGit]

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[Standard8]

As discussed elsewhere, the fix here is correct, however due to our lack of fallback (#[serde(other)]), this won't work for existing clients that have already been shipped.

I think we should delay this until we need search-config-v3. In the meantime the relevant engine could be switched to "unknown".

Setting to "request changes" to clear from review queue.

[Standard8]

The types that I put in previously were intended as examples, rather than exact definitions. I don't think we want to put ourselves in a situation of having to update documentation that's defined elsewhere, so how about something like:

/// The classification of search engine according to the main search types
/// (e.g. general, shopping, travel, dictionary; for exact definitions see `SearchEngineClassification`).