BlobStore for retrieving raw blob data from the db

[sandreae]

caveat: I'm still on the fence about whether we want to store blobs like this (all pieces as their own document) but I wanted to try out this approach first as a) it fits so smoothly in with our current documents system b) it's easy to do and the api won't really change if we take a different approach to storing blobs anyway. So, this is the minimal-changes approach, and we can iterate from here 👍

Introduces BlobStore with one method: get_blob.

Under the hood this:

• gets the target document by id
• checks it's schema id is blob_v1
• fetches all related pieces in it's pieces field
• validates that all pieces have been collected
• combines them into one string
• validates that the length matches the claimed length on the blob document
• returns this string

This is done without introducing a new Blob type into p2panda-rs (as was the approach with Schema, our other system schema type). I actually think this works ok for now, but interested in any thoughts on this.

📋 Checklist

• Add tests that cover your changes
• Add this PR to the Unreleased section in CHANGELOG.md
• Link this PR to any issues it closes
• New files contain a SPDX license header

[codecov]

Codecov Report

Patch coverage: 94.41% and project coverage change: +0.01% 🎉

Comparison is base (d5ba55d) 91.95% compared to head (76f40ec) 91.97%.
Report is 2 commits behind head on development.

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #484      +/-   ##
===============================================
+ Coverage        91.95%   91.97%   +0.01%     
===============================================
  Files              100      101       +1     
  Lines            15914    16110     +196     
===============================================
+ Hits             14634    14817     +183     
- Misses            1280     1293      +13     

Files Changed	Coverage Δ
aquadoggo/src/db/errors.rs	0.00% <0.00%> (ø)
aquadoggo/src/db/stores/blob.rs	94.87% <94.87%> (ø)
aquadoggo/src/schema/schema_provider.rs	89.93% <100.00%> (ø)

... and 2 files with indirect coverage changes

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