Add tests for custom functions

[zimeg]

To ensure stable logic and proper error handling in custom functions, tests should be added with mocks of API endpoints.

These tests should cover the happy path and ensure APIs are called with the correct data the appropriate number of times (for example, conversations.join should be called for each listed channel in the maintain_membership.ts function). Error handling should also be tested, particularly for external auth related functionality.

After adding tests, a section on testing should be added to the README.