feat(sdk): add Docker backend support to TypeScript and Python SDKs

[spichen]

This commit adds Docker backend support to both the TypeScript and Python SDKs:

TypeScript SDK:

• Add BackendType type for backend selection (wasmer, firecracker, docker, auto)
• Add DockerOptions interface for Docker-specific configuration
• Add backend option to BashletOptions and ExecOptions
• Update client to pass --backend flag to CLI
• Export new types from index
• Add tests for Docker backend functionality

Python SDK:

• Add BackendType enum for backend selection
• Add DockerOptions dataclass for Docker-specific configuration
• Add backend option to BashletOptions and ExecOptions
• Update both sync and async clients to support backend parameter
• Support both string and enum values for backend
• Export new types from init.py
• Add tests for Docker backend functionality

Users can now specify the Docker backend when executing commands:

TypeScript:
const bashlet = new Bashlet({ backend: 'docker' });
await bashlet.exec('ls', { backend: 'docker' });

Python:
bashlet = Bashlet(backend='docker')
bashlet.exec('ls', backend=BackendType.DOCKER)