Python plugin: add faucet/reward tx support + custom RPC endpoints

[demarco2016]

Implements the Python SDK counterpart of the Go custom RPC endpoint support (PR #414).

Changes (5 files in plugin/python/):

contract/proto/tx.proto + tx_pb2.py

• Added Faucet, Reward, MessageFaucet, MessageReward proto messages

contract/contract.py

• Added prefixes 0x64/0x65 (outside core reserved range 1-15)
• Key generators: key_for_faucet / key_for_reward
• check_tx / deliver_tx dispatch for faucet and reward messages
• Registered custom state prefixes in CONTRACT_CONFIG
• _check_message_faucet, _check_message_reward stateless validation
• _deliver_message_faucet, _deliver_message_reward stateful execution

contract/rpc.py

• Route dispatch for /v1/query/faucets and /v1/query/rewards
• Handlers using asyncio.run_coroutine_threadsafe
• Supports single-address exact lookup (?address=hex) and full prefix scan

Matches the Go plugin behavior:

• Admin penalty on reward (fee deducted from admin)
• Detached read-only queries (no tx lifecycle coupling)
• Collision-safe custom prefixes (0x64/0x65)