feat: eliminate Go sidecar, send gRPC events directly to webhook plugin

[VivekPipaliya23]

Replace the vernemq-events-sidecar Go service (TCP protobuf → gRPC translator) with direct gRPC calls from the Erlang plugin to the webhook plugin using emqx/grpc-erl. This removes a network hop and an entire service from the deployment, while keeping the webhook plugin unchanged.

Key changes:

• Replace shackle TCP client with grpc-erl gRPC client (grpc_client:unary/4)
• Add full webhook service proto (PluginService.Event) to vmq_proto
• Send user-type as gRPC metadata header instead of sidecar config
• Fire-and-forget via spawn/1 around synchronous gRPC call
• Update config schema: grpc_endpoint, grpc_port, grpc_pool_size, grpc_timeout, user_type replace hostname, port, pool_size, backlog_size
• Rewrite test infrastructure: gRPC server handler replaces TCP server

Proposed Changes

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