T8382: Add VPP Prometheus exporter support

Integrate the VPP Prometheus exporter with VyOS configuration. Upon
being configured via VyOS CLI, the exporter reads VPP statistics and
exposes selected metrics at /metrics endpoint for Prometheus to
scrape. VPP exporter service can co-exist with node-exporter (both
independently) to export VPP and kernel stats respectively.

The implementation provides functionality:

CLI configuration for the exporter, including its listen port, VRF,
and selectable stat groups: interfaces, err, buffer-pools, system,
workers, nodes, memory, nat44, and acl.

Default stat groups of interfaces, err, buffer-pools, system, workers,
and memory. Explicitly selecting any groups replaces that default set.

The exporter is opt-in: configuring vpp-exporter enables and starts
it. Removing the configuration stops the exporter independently of
VPP. While configured, it follows VPP’s lifecycle and restarts when
its own configuration changes.

Validation that requires VPP to be configured before configuring the
exporter, and rejects VPP removal while the exporter remains
configured.

Node metrics through the nodes group when VPP’s per-node-counters
setting is enabled. Selecting nodes without that setting produces a
warning. Removing the exporter leaves per-node-counters unchanged.

VRF support for the exporter service.

Co-authored-by: @inetman28
PR-Ref: #5339 (Addressed review comments, removed regex CLI options)

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