Enhance user features: mylists, series, and activity

[Shi-553]

This pull request introduces several new features and improvements to the NicoNico Python client, focusing on user interactions, mylist management, and activity/feed retrieval. The main changes include new API methods for following/unfollowing users, enhanced mylist operations (such as adding/removing items and creating lists), support for fetching user activities and feeds, and improved data modeling for these features.

User interaction and following features:

• Added follow_user and unfollow_user methods to UserClient, enabling users to follow or unfollow other users via the NicoNico API. This required the addition of a generic delete method to the HTTP client for DELETE requests. [1] [2]

Mylist management enhancements:

• Introduced new methods to UserClient for mylist operations: get_own_mylist, add_mylist_item, remove_mylist_items, and create_mylist, allowing users to fetch, modify, and create mylists programmatically. Corresponding data models such as CreateMylistData were added. [1] [2] [3]

Feed and activity retrieval:

• Added support for retrieving user feed activities and following lists/tags with methods like get_own_following_mylists, get_own_following_tags, and get_following_activities, along with new data models for feed and activity responses. [1] [2]

Data model improvements and fixes:

• Fixed field aliasing in data models for better compatibility with API responses, such as using Field(..., alias="sampleItems") in UserMylistItem and defaulting optional fields in SeriesOwner. [1] [2]

Search API improvement:

• Updated search_lists to accept a sort_order parameter, allowing users to specify sort direction when searching lists. [1] [2] [3]