feat: add grpc-disabled config option for server

[kalleep]

What this PR does:
Add config option to disable the grpc listener when creating a Server. I named it GRPCDisabled to keep it consistent with other grpc related options.

With this option the grpc listener is never created. The GRPC server is still created but will never be started, we could change this too if that is a preference.

Which issue(s) this PR fixes:

Fixes #954

Checklist

• Tests updated

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Note

Medium Risk
Changes core server startup/run behavior and introduces conditional gRPC initialization, which could impact deployments relying on gRPC being available or on side effects of HTTP-over-gRPC registration.

Overview
Adds a new Config.GRPCDisabled (YAML grpc_disabled + -server.grpc-disabled flag) to optionally skip creating the gRPC listener.

When enabled, server startup now avoids gRPC listener setup (including PROXY protocol wrapping), adjusts startup logging, and skips registering/serving the HTTP-over-gRPC handler; GRPCListenAddr() returns nil in this mode. Adds TestGRPCDisabled to verify HTTP still serves while gRPC is not listening.

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