Add sessions subpackage

[vdusek]

Description

• Introducing two new classes: Session & SessionPool.
• The state of the SessionPool can be persisted into KVS, Pydantic models are utilized for this case.
  • Storing cookies and user data as dictionaries. Initially, I considered using a CookiesType union with http.cookie_jar.CookieJar, httpx.Cookies, and dictionary, but these types were not serializable by Pydantic. However, both can be converted to dictionaries, so there should be no problem with cookies being a dict type.
• There have been slight modifications to the EventManager, which now functions as a context manager (same as LocalEventManager).
  • Additionally, I enhanced its test coverage.

Issue

• Implement session management #78

Tests

src/crawlee/sessions/__init__.py                                        2      0   100%
src/crawlee/sessions/models.py                                         54      5    91%
src/crawlee/sessions/session.py                                        89      2    98%
src/crawlee/sessions/session_pool.py                                  133     20    85%

[janbuchar]

The implementation is super nice, I like how you split the (de)serialization from the business logic. I found a bunch of things to nitpick though 🙂