VPP: T7175: Conf mode CLI and startup template.

[ServerForge]

Change Summary

Added conf mode CLI for VPP sflow plugin and updated VPP template to include plugin.

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)
• Other (please describe):

Related Task(s)

https://vyos.dev/T7175

Related PR(s)

vyos/vyos-build#962

Proposed changes

Added conf mode CLI for VPP sflow plugin and updated VPP template to include plugin. It allows configuring the VPP interfaces that will be sampled and the sample rate. Currently only ingress sampling is supported by the plugin. There is an additional setting that needs to be flipped under the "system sflow" that will tell it to poll the VPP plugin and has a PR active under the vyos-1x repo.

vyos@vyos# show vpp sflow 
 interface eth0
 interface eth1
 sample-rate 5588

How to test

Checklist:

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