interfaces: T4885: add 'clear interfaces counters' to op-mode

[jestabro]

Change Summary

Note: this will need to be merged after
vyos/vyatta-op#59
to avoid conflicting templates.

The functions clear_counters and reset_counters were originally translated from Perl to Python in operational.py, however, they had errors and were never exposed in op-mode. Add fix; add to op-mode interfaces.py; add op-mode-defs.

Note that reset counters was never exposed in vyatta-op: the 'reset' function removes the cached values (from a previous clear), returning to the raw counters of '/sys/class/net/ifname/statistics'. It is a question as to whether this is useful; this PR fixes the reset function but does not expose it, focusing only on the useful 'clear' function. Reset is trivial to add if desired.

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/T4885

Component(s) name

Proposed changes

How to test

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