M.D.Sqlite: More robust cross-platform extension loading

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Our current (native) extension loading story for Sqlite isn't great: it's usually up to the user to make sure that the native DLL/so/dylib is present in the operating system search path, etc. (e.g. spatialite docs). This notably makes the getting started story for popular extensions like sqlite_vec much more difficult than it should be.

Thanks to @krwq for working on improving this.

Note: update the spatialite docs.

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Keeping open to track backporting to 8.0 and 9.0, for a better sqlite_vec story.