Introduce the dm API

[jadolg]

At some point the im API was renamed to dm.
The methods are the same, therefore the tests are a verbatim copy and only the methods names change.

I'll discontinue the im methods in a future version.

Follow this thread for more context: https://open.rocket.chat/channel/python_rocketchat_api?msg=j8AyPJRk8cLQYv7kj

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.74%. Comparing base (5001fac) to head (0fbaa13).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #325      +/-   ##
==========================================
+ Coverage   97.57%   97.74%   +0.16%     
==========================================
  Files          43       45       +2     
  Lines        1937     2082     +145     
  Branches      107      113       +6     
==========================================
+ Hits         1890     2035     +145     
  Misses         35       35              
  Partials       12       12              

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[sonarqubecloud]

Quality Gate failed

Failed conditions
9.9% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud

[jadolg]

Quality Gate failed

Failed conditions 9.9% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud

We're fine with duplicating this code. It should be exactly the same. The new API is supposed to replace the old one at some point anyway