[RFC] PAM: Build GDM support only on the library

[3v1n0]

There's quite a bit of code that is GDM related that we don't really need to include into the compiled PAM binary that we use for authentication in non-GDM cases.

We don't really use all the gdm-related code in the PAM Exec child that we use in normal PAM transactions, so no need to compile it at all by default while we need it in the PAM library that GDM will consume.

This is basically the opposite of #292, as that tried to unify the logic while here we assumed that we're going to ship two different implementations, and so reducing the logic in one side when is not needed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.

Project coverage is 84.11%. Comparing base (87a34c4) to head (d219a3e). Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pam/internal/adapter/gdmmodel.go	71.42%	2 Missing :warning:
pam/internal/adapter/gdmmodel_disabled.go	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #566      +/-   ##
==========================================
+ Coverage   83.77%   84.11%   +0.34%     
==========================================
  Files          80       82       +2     
  Lines        7098     7077      -21     
  Branches       75       75              
==========================================
+ Hits         5946     5953       +7     
+ Misses        820      791      -29     
- Partials      332      333       +1     

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[3v1n0]

I've made the integration tests to require the gdm tag now and added to debian side too