[BinFmt] Opt-in warn on Custom BuildEventArgs usage

[JanKrivanek]

Background

#6215
This subitem is focused on FullFw usages of custom BuildEventArgs.
Custom BuildEventArgs currently uses BinaryFormatter for de/serialization: https://github.com/dotnet/msbuild/blob/main/src/Shared/LogMessagePacketBase.cs#L377 This is going to be resolved for Core as in #8823. The fixing PR introduces warning for Core in case the Custome BuildEventArgs are attempted to be serialized (here and here). In order to allow easier depart of Custom BuildEventArgs for FullFw builds - we want to introduce opt-in mechanism issuing same warnings.

Goal

Same warnings as introduced in #8917 (see links above for precise location) can be opted-in for FullFw build

Suggested approach

Define global property that will enable #ifdefed code introduced in the mentioned PR.
Create FullFW unit test verifying the warning is issued