[mdatagen] expose host in generated test code by mdatagen

[atoulme]

Description

Expose a setting on tests::host to set up your own host initialization code

Some receivers require a host that has additional capabilities such as exposing exporters.
For those, we can expose a setting that allows them to place a different host in the generated code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.11%. Comparing base (524a901) to head (bc09900).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10765   +/-   ##
=======================================
  Coverage   92.11%   92.11%           
=======================================
  Files         403      403           
  Lines       18806    18806           
=======================================
  Hits        17323    17323           
  Misses       1123     1123           
  Partials      360      360           

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