[improve][fn] Set default tenant and namespace for ListFunctions cmd

[jiangpengcheng]

Fixes #23880

Main Issue: #xyz

PIP: #xyz

Motivation

The list sources and sinks command will use default tenant and namespace when not specified, the list functions cmd should have the same bahaviour

Modifications

Verifying this change

• Make sure that the change passes the CI checks.

• This change added tests and can be verified as follows:

  • Added unit test for list functions to use default tenant and namespace

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
  • when list functions using pulsar-admin without tenant and namespace, no error will be thrown, but the default tenant public and namespace default will be used
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: jiangpengcheng#38