Add YAML configuration model and loader

Config is layered: built-in defaults, then the YAML file, then BFD_* environment variables, then CLI flags. Global defaults live under a 'defaults' mapping; per-peer settings are file-only. Includes config.yaml.dist and ScriptHookSink::with_script_dir so the hooks directory is configurable.

Cargo.toml

@@ -7,9 +7,11 @@ edition = "2021"
 atomic = "0.6.0"
 bytemuck = { version = "1.14.1", features = ["derive"] }
 byteorder = "1.5.0"
-clap = { version = "4", features = ["derive"] }
+clap = { version = "4", features = ["derive", "env"] }
 env_logger = "0.11.1"
 futures = "0.3.30"
+humantime = "2.4.0"
+humantime-serde = "1.1.1"
 itertools = "0.12.1"
 log = "0.4.20"
 nix = { version = "0.31.3", features = ["fs", "socket"] }
@@ -17,5 +19,7 @@ nom = "7.1.3"
 nom-derive = "0.10.1"
 proc-bitfield = "0.3.1"
 rand = "0.8.5"
+serde = { version = "1.0.229", features = ["derive"] }
+serde_yaml_ng = "0.10.0"
 tokio = { version = "1.35.1", features = ["net", "full"] }
 tokio-timer = "0.2.13"