Support for .skyignore or SYNC_FILE_MOUNTS in sky.exec

[Danivilanova]

When using sky.exec, it lacks support for file_mounts unlike sky.launch, limiting file synchronization flexibility. Our local .gitignore prevents necessary files from syncing.

Suggested Solution

Introduce a .skyignore file for file sync exclusions.
Add a SYNC_FILE_MOUNTS stage to sky.exec to enable file_mounts usage.