Add support for async close on http client

[logan-twilio]

Fixes #314

Adds close() and async context manager support to AsyncClient and ClientBase, giving users a reliable way to release the underlying aiohttp.ClientSession without depending on __del__.

New usage patterns:

# Async context manager
async with stytch.Client(project_id=..., secret=...) as client:
    await client.magic_links.email.login_or_create(...)

# Explicit close (e.g. in a FastAPI lifespan)
client = stytch.Client(project_id=..., secret=...)
try:
    ...
finally:
    await client.close()

close() is a no-op if the session was externally provided or never created. The existing __del__ fallback is preserved for cases where neither pattern is used.