Secret dir inside alloc dir to share sensitive files between tasks

### Proposal
Like a secret directory inside the task level directory, create a secret directory inside the alloc directory so tasks can share sensitive files using the alloc/secret directory and hide this directory from UI/API simultaneously.

### Use-cases
My use case is, we have a tool to generate certificates using pre-start tasks and store files in the alloc/secret directory, and my main task is that the Nginx container can consume these files for terminating tls connections.
Currently, we are storing them in the alloc/data folder, but these files are visible in UI & CLI. We can hide them using ACL, but we don't want to hide all files from viewing and couldn't find a way to hide specific files.

Discussed this here: https://discuss.hashicorp.com/t/how-to-share-sensitive-files-between-tasks-but-hide-from-ui/46630

Start by tracing how allocation directories are created and how alloc/data files are exposed through the UI and API. Review the linked discussion for the sensitive-file use case and expected visibility behavior. Done means tasks can share files in alloc/secret while those files remain hidden from the UI and API without hiding other allocation files.

Written by the indexing model from the issue text.