feat: Adds secondary sqlite db to ads-client

[thesuzerain]

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

This adds an 'adstore' equivalent to the to be used for storing ads in the new stateful ads-client design. It does not provide any FFI or interfaces or hooks or anything, simply the sqlite queries and internal logic, mimicking a lot of the logic for the http_cache. This uses a separate sqlite db to the http_cache one, though they are functionally quite similar. The tests are all similar/the same, except for removed ones (such as TTL tests, see below).

In particular, this does not include:

• In-memory database fallbac: Waiting on confirmation for this, but this currently simply stores the AdsStore as None if the proper db path is not provided n setp. The in-memory db can be added in another PR after as a default.
• TTL: Because the TTL strategy will look different for this than the http_cache (we will have a background worker thread updating + clearing) it's not included in this PR
• Expanded telemetry (eg: http_cache has record_http_cache_outcome for cache hits/misses)

I did, however, keep:

• bytesize (as I imagine we will be keeping the max_size part)
• clock + stored_at (as this will almost certainly be relevant to TTL later)

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[thesuzerain]

This struct isn't strictly needed at this moment, but it makes the test generation a bit cleaner, and I could see it being useful for the next PR in the process (which will likely utilize conversions from AdImage, etc, to a more generic structure- this one)