Fix `arbitrary` feature flag plumbing so it compiles

[CBenoit]

The arbitrary feature is currently broken / not consistently compiled. We want it working again and wired correctly across crates that define PDUs.

Scope

• Add/repair arbitrary feature flags in crates that own PDU structs/enums
• Ensure the dependency is optional and feature-gated
• Ensure it works in both std and no_std + alloc contexts where applicable
• Document “how to build with arbitrary” in the relevant .md documents

Tasks

• Identify target crates (likely ironrdp-pdu and any other protocol crates that expose PDU types)
• Add arbitrary = { version = "1", features = ["derive"], optional = true }
• Add features wiring (arbitrary = ["dep:arbitrary", ...])
• Check cargo check -p <crate> --features arbitrary passes locally

[dependencies]
arbitrary = { version = "1", features = ["derive"], optional = true }

[features]
# Pick one:
# - If the crate is std-only: arbitrary = ["std", "dep:arbitrary"]
# - If the crate supports no_std + alloc: arbitrary = ["alloc", "dep:arbitrary"]
arbitrary = ["alloc", "dep:arbitrary"]

Acceptance criteria

• cargo check for the target crates succeeds with --features arbitrary
• No breaking change for default feature sets