Add existing sync target

[damienfamed75]

Adds existing sync target that generates from existing lockfile entries without upload any assets. This is useful for CI linux runners to generate a production-ready asset file without uploading.

Also if any asset's hash is not found in the lockfile the command fails with an error

usage: asphalt sync existing

[jackTabsCode]

Hey. I'm not completely sold on adding this target-I think it's a bit strange to have a separate one from Cloud that uses the lockfile, which only Cloud uses.

Can you let me know what you gain from not checking in your generated asset files? This is the standard workflow and also makes your code typecheckable without needing an extra step.

[damienfamed75]

Can you let me know what you gain from not checking in your generated asset files? This is the standard workflow and also makes your code typecheckable without needing an extra step.

My use for this may be a bit specific. Assets are git-ignored and whenever testing locally we use the studio generated asset file so type safety always works. Once changes are merged, there's a pipeline that generates the asset file with the public rbx id's, then a rbxl is built and uploaded to a test place.
Before merging there's a workflow that uses the existing target to test for any new assets that haven't been cloud synced yet.

Hey. I'm not completely sold on adding this target-I think it's a bit strange to have a separate one from Cloud that uses the lockfile, which only Cloud uses.

If it makes more sense, I can make it an option for the cloud target instead