[AC-154] implement fire and forget ads client async

[thesuzerain]

This is a draft PR and not yet ready for review.

Discussion points:

I've excluded the following from this PR:

• The nimbus work, while I will implement in a separate PR. We do want to use nimbus for the various slices of the async work, but because this adds an entirely new set of surface entrypoints, the branching based on nimbus flags will need to happen on the surface layer. (That being said, future slices will be able to use this, so it will still be included, but will be included separately)
• In an earlier version of this PR, I included a callback ErrorOnlyCallback that would allow the surfaces to receive any errors from the process running in the worker thread. However, in light of the uniffi callbacks issue, it requires a bit of refactoring to ensure these can be passed to the thread and shutdown successfully- possible, but adds a lot to this PR, so I've excluded it for now.
  • As a result, if there are errors in the fire-and-forget functions like RecordClick, the tests currently do not catch them.

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