feat(types): allow custom beta header values

[MukundaKatta]

Summary

• loosen SdkBeta so callers can pass newly released anthropic-beta header values without waiting for an SDK release
• keep the existing known beta literal discoverable while allowing arbitrary strings
• add a transport test that verifies custom beta values still flow through to the CLI --betas flag

Testing

• python3 -m py_compile src/claude_agent_sdk/types.py tests/test_transport.py
• python3 -m pytest tests/test_transport.py -k custom_betas (fails in this environment because the test import path is missing the mcp dependency)