op-mode: T6566: add support for listing all interfaces in "monitor bandwidth" (backport #3805)

[mergify]

Change Summary

Right now we can only monitor the bandwidth for one individual interface, but not all at once. This adds support to monitor all interfaces.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T6566

Related PR(s)

Component(s) name

bmon

Proposed changes

How to test

[email protected]:~$ monitor bandwidth
Possible completions:
  <Enter>               Execute the current command
  interface             Monitor bandwidth usage on specified interface


[email protected]:~$ monitor bandwidth

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

---

This is an automatic backport of pull request #3805 done by [Mergify](https://mergify.com).

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