New Rule: ExcludeFromCodeCoverage attributes should include a justification

[martin-strecker-sonarsource]

Both reasoning and implementation are similar to RSPEC-1123: "Obsolete" attributes should include explanations.

Implementation: #6593
RSpec: SonarSource/rspec#1624

[martin-strecker-sonarsource]

Peach validation: Found 9 true positives on dotnet-runtime. All had a justification as a code comment next to it like e.g.

https://github.com/dotnet/runtime/blob/7f0c19acfcfe1d89d1aaee814e6fd00b3ff1c8a2/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableHashSet_1.NodeEnumerable.cs#L51

The Justification property was not used because of multitargeting (..;netstandard2.0;..):
https://github.com/dotnet/runtime/blob/7f0c19acfcfe1d89d1aaee814e6fd00b3ff1c8a2/src/libraries/System.Collections.Immutable/src/System.Collections.Immutable.csproj#L3