merklize: Support masking unix_mode when building merkle trees

Before raising this MR, consider whether the following are required, and complete if so:

• Unit tests
• Metrics
• Documentation update(s)

Description

Support masking unix_mode (permissions) when building merkle trees.

For now, buildbox tools never set unix_mode and it's up to the worker sandbox to reconstruct the appropriate file permissions.

To support more flexible use cases, e.g., workspaces must be writable, we can add a callback when constructing merkle trees to override permissions.

Changes proposed in this merge request:

• Add unixModeUpdater in make_nesteddirectory and File constructor.
• Dangerous permissions, i.e. other and special, have to be explicitly added. IIRC, before this change, they can never be turned on if using both buildbox uploading tools and runners.
• The original unix_mode can be captured if unix_mode is in the property set.