Add grammar-based payload generation via barkus

[GeorgeHahn]

Summary

• Integrates barkus as a new grammar payload type in lading_payload
• Enables any existing generator (TCP, UDP, HTTP, file, unix socket, etc.) to send grammar-generated structured data from EBNF, PEG, or ANTLR v4 grammar files
• Bridges rand 0.9 (lading) to rand 0.10 (barkus) by seeding a local SmallRng, preserving determinism
• Validates at construction time that the grammar is reachable within the configured depth budget
• Caps consecutive generation failures to prevent infinite loops on pathological grammars

Config example

generator:
  - tcp:
      seed: [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1]
      addr: "127.0.0.1:8080"
      variant:
        grammar:
          grammar_path: /path/to/json.ebnf
          format: ebnf
          max_depth: 40
      maximum_block_size: "1 MiB"
      maximum_prebuild_cache_size_bytes: "8 MiB"