[WIP] Adding shared alloc/secrets dir

[angrycub]

Add a secrets dir that can be shared across tasks in a single allocation.

TODOs

• Implement shared alloc secrets dir
  • Make secrets dir in alloc root
  • Bind secrets dir into tasks
  • Validate secrets are read/write between tasks
• Update taskenv to support new directory
• Test with different task drivers
  • raw_exec (unisolated)
  • exec (namespace isolation)
  • qemu (image isolation)
• Add documentation

Fixes: #15249