Snoop: change workload dump command to print to stdout

Context

Resolves #5023 (closed)

With this MR,

• ./octez-snoop workload dump add.workload outputs the content to the stdout, and
• ./octez-snoop workload dump add.workload -o add.json saves to add.json.

$ ./octez-snoop man
...

Commands for manipulating workload files:
  workload dump <WORKLOAD-FILE> [-o --out-file <OUTPUT-FILE>]
    Dump the content of a workload file in JSON format
    <WORKLOAD-FILE>: Workload file name
    -o --out-file <OUTPUT-FILE>: JSON file name to write the content

...

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR