refactor(storage/fs): change fs.FSSource into fs.SnapshotSource

[GeorgeMac]

This refactors the fs.Store implementation to work over a new type fs.SnapshoutSource instead of fs.FSSource.

The difference between these abstractions is instead of returning fs.FS interface implementations, this source returns the full snapshots. The benefit here is the source can decide how to build the snapshot. Instead of having to conform to fs.FS. The storage fs.SnapshotFromFS can still be used to turn any fs.FS into a snapshot.

All the existing sources have just been updated to call this utility function to adapt their existing fs.FS implementations.
Future implementations don't have to use this abstraction and we can change existing ones if and when it makes sense.

The last thing it does is expose a new SnapshotFromFiles method.
This method just takes a slice of fs.File instead of the full fs.FS.
The fs.File interface is actually quite appropriate and more approachable than the full fs.FS abstraction.
It will be necessary for both reading contents and for identifying file extension (to support both Yaml and JSON).

The OCI implementation will skip the fs.FS abstraction and use just the fs.File implementation instead.
I can update all the open FS issues to reference these new abstractions instead.

[github-actions]

Uffizzi Preview deployment-39955 was deleted.

[markphelps]

love this abstraction!

[codecov]

Codecov Report

Merging #2325 (06cc939) into main (85e5f3a) will increase coverage by 0.05%.
The diff coverage is 75.32%.

@@            Coverage Diff             @@
##             main    #2325      +/-   ##
==========================================
+ Coverage   70.87%   70.92%   +0.05%     
==========================================
  Files          78       78              
  Lines        7436     7449      +13     
==========================================
+ Hits         5270     5283      +13     
+ Misses       1866     1865       -1     
- Partials      300      301       +1     

Files	Coverage Δ
internal/storage/fs/local/source.go	91.17% <100.00%> (+0.26%)	⬆️
internal/storage/fs/store.go	94.00% <100.00%> (+14.68%)	⬆️
internal/storage/fs/s3/source.go	76.71% <70.00%> (-2.70%)	⬇️
internal/storage/fs/git/source.go	79.77% <61.53%> (-2.16%)	⬇️
internal/storage/fs/snapshot.go	70.28% <71.79%> (-0.05%)	⬇️

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