feat: add gRPC/OpenConfig telemetry as a data input method

[somethingwithproof]

Description

Several major network vendors (Juniper, Cisco, Arista, Nokia) now support
gRPC-based streaming telemetry via OpenConfig or vendor-native interfaces
as their primary monitoring path. This is increasingly preferred over SNMP
for high-frequency metric collection on modern platforms.

A native gRPC data input method in Cacti would allow:

• Streaming telemetry subscriptions (dial-in or dial-out)
• Higher poll frequency without SNMP overhead
• Access to metrics not exposed via SNMP MIBs
• Better compatibility with devices deprecating SNMP support

Vendor Coverage

• Juniper: JTI (Juniper Telemetry Interface), OpenConfig
• Cisco: MDT (Model-Driven Telemetry), gNMI
• Arista: OpenConfig, gNMI
• Nokia: gRPC/gNMI

Implementation Considerations

• PHP gRPC extension (pecl grpc) or a sidecar collector that writes to
  Cacti's poller_output table
• Protobuf schema handling for vendor-specific and OpenConfig models
• Integration with existing RRD storage and graph rendering pipeline
• Could start as a plugin before core integration

Related

• Cacti should support with bogus OIDs coming from devices (example: Juniper Netscreen security metrics) #6108 — Juniper Netscreen OID index padding (workaround for SNMP limitations
  on devices moving toward gRPC)

[somethingwithproof]

Context: #6108 documents a concrete example of SNMP MIB limitations on Juniper devices that gRPC telemetry would bypass entirely.

[somethingwithproof]

Tracked in #7034 (polling engine roadmap).