API v1.1 Direct Message endpoint wrappers

[alkihis]

Provide wrappers and paginators to nearly every public DM endpoints (in 1.1 API):

• Send, get, list and delete DM events
• Send, get, list, update and delete welcome DMs
• Create, get, list and delete welcome DM rules (+ helper to combine list+delete+create)
• Mark a DM as read
• Indicate typing in a DM 1-to-1 conversation

With appropriate documentation + test suite :)

Note: Test suite is disabled by default, as it requires a user-defined sender ID and an application that supports direct messages auth level. To enable test suite, define env variable TARGET_DM_USER_ID.

[PLhery]

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