[#7] Cache slack data

[YuriRomanowski]

Description

Problem: The bot uses two methods, getUser and getChannelMembers, which are subjects to Slack's rate limits, and we need to avoid unneeded calls to those methods.

Solution: Add caching for user info and channel members.

Related issue(s)

Fixed #7

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Related changes (conditional)

• Tests

  • [ ] If I added new functionality, I added tests covering it.
  • [ ] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • [ ] I checked whether I should update the docs and did so if necessary:
  • README
  • Haddock

Stylistic guide (mandatory)

• [x] My commits comply with the policy used in Serokell.
• [x] My code complies with the style guide.

[YuriRomanowski]

Guys how do you think, do we need some tests here for the actual events processing and caching?

[alexd1971]

Probably it would be better to rebase this branch onto main before review.