Add +rpc cabal flag to gate gRPC dependencies

[carbolymer]

Summary

The cardano-rpc package introduces new dependencies (grapesy, grpc-spec, proto-lens) that are currently linked into all cardano-node builds, including block producers that will never enable the RPC feature.

A cabal flag (+rpc, off by default) should gate these dependencies so that builds without the flag exclude them entirely.

Motivation

• Block producer builds should not carry unnecessary dependencies that increase the attack surface and binary size.
• Operators who want the RPC interface opt in explicitly at build time.
• This was requested during ADR-018 review.

Proposed approach

• Add a flag rpc (default: False, manual: True) to cardano-node.cabal.
• Make build-depends: cardano-rpc conditional on flag(rpc).
• Guard RPC server startup code in cardano-node behind a CPP conditional (e.g. #ifdef RPC).
• Document the flag in the README and in ADR-018.

Acceptance criteria

• cabal build cardano-node (without the flag) does not pull in grapesy, grpc-spec, proto-lens, or cardano-rpc.
• cabal build cardano-node -f +rpc builds with full RPC support.
• CI runs builds with and without the flag.

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