What would you like to be added:
Ability for user to specify a specific snap revision to pull.

Why is this needed:
The snap channels are a moving target, a user might want to specify a specific snap revision, perhaps using something like:

syft --from snap snap-name@revision

... though, name@value currently specifies the channel, so the specific syntax or additional flags needed are TBD.

Additional context:
Snap tool uses: snap download name --revision 2379

Snap API docs are: https://api.snapcraft.io/docs/info/