Improve argument parsing of the mqtt loader

[twiggler]

This PR improved upon the argument parsing of the mqtt loader.
More specifically,

1. It checks for invalid values. For example, check that the specified number of peers is strictly positive
2. It adds more helpful error messages.

Additionally,

1. It deletes the legacy targetd loader (Targetd: Improve argument parser MQTT -loader and remove old Targetd-loader #849 was about two things, now not entirely sure how they are related).
2. Fixes a typing issue.
3. Removes the hostname prefix of CALLID messages
4. Simplify mocking of MQTTClient

Closes #849

[codecov]

Codecov Report

Attention: Patch coverage is 51.61290% with 15 lines in your changes missing coverage. Please review.

Project coverage is 76.19%. Comparing base (765c668) to head (dc1ed23).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dissect/target/loaders/mqtt.py	51.61%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #850      +/-   ##
==========================================
+ Coverage   75.89%   76.19%   +0.29%     
==========================================
  Files         312      310       -2     
  Lines       26842    26674     -168     
==========================================
- Hits        20372    20323      -49     
+ Misses       6470     6351     -119     

Flag	Coverage Δ
unittests	76.19% <51.61%> (+0.29%)	⬆️

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[pyrco]

Probably better to remove the host/ip checks. I don't think it is worth the aded complexity as the broker connection could fail due to a number of reasons anyway.