Add a custom rule identifier.

[YuliiaKovalova]

There is a need to understand if a rule belongs to custom category or is a build-in for .editorconfig purposes.
I suggest to create a child class CustomBuildAnalyzer.cs for custom analyzers from

msbuild/src/Build/BuildCheck/API/BuildAnalyzer.cs

Line 14 in c305c69

/// <see cref="BuildAnalyzer"/> is a unit of build analysis execution, but it can contain multiple rules - each representing a distinct violation.

and restrict on this type during instantiation : https://github.com/dotnet/msbuild/blob/c305c6901b2bf74867efcb8b6c5f3083ef81c30b/src/Build/BuildCheck/Acquisition/BuildCheckAcquisitionModule.cs#L45C1-L46C1

[YuliiaKovalova]

Seems to be not needed for now.