Flexible schedule for sending daily tasks and version update

• Now subscribers can choose an hour to receive daily tasks.
• Go module version updated to v3.

v3.0.1

• bug fixes in storage/ydb
• new tests for storage/ydb
• database schema change

New Leetcode API version, context and tests

• At October 1 Leetcode changed API for daily challenges. So there is no more itemID-links and titleSlug working as ID. Also separate interface of daily challenges gone away, so URLs also new.
• Leetcode client and whole bot app respect context now.
• Test coverage significantly increased from prev version.
• Code climate rate is A now and no issues left.

Task difficulty hints

• Added task difficulty hints for tasks.
• Readme updated with example of using client library.
• Removed old python version.

First golang version

Successor of python version. Much faster on initialization and in general. Also Golang package management more mature from my point of view, so client library could be reused bit easily.

Stable python version

Stable python version. By some reason initialization of python functions in Yandex.Cloud took up to 700-800ms, which is significant.
So this is the last version with python.