Release Screenshot API support in v0.2.0

[fatwang2]

What changed

• add synchronous and asynchronous Screenshot API methods
• return binary image content, content type, and request ID
• expose Screenshot API examples in the README
• bump the package to 0.2.0 and update the client identification header
• make Retry-After date parsing explicitly typed across Python targets

Validation

• Ruff format and lint passed
• mypy passed for Python 3.9 and 3.12 targets
• 10 tests passed
• sdist and wheel builds passed
• Twine package checks passed