[mono] Share more of the CustomAttribute logic between CoreCLR and Mono

[ivanpovazan]

To improve code quality and maintainability and have the same behaviour between the runtime flavours we should try to share more code/logic between CoreCLR's and Mono's implementation of CustomAttribute API.

This has been called out in: #94602 and should try to address code comments used with Mono:

• runtime/src/mono/System.Private.CoreLib/src/System/Reflection/CustomAttribute.cs

  Line 114 in 2454fe6

  // FIXME: Callers are explicitly passing in null for attributeType, but GetCustomAttributes prohibits null attributeType arguments

• runtime/src/mono/System.Private.CoreLib/src/System/Reflection/CustomAttribute.cs

  Line 469 in 2454fe6

  //FIXME resolve this case if it makes sense. Assign empty array for now.

• and others ...

to share/match CoreCLR's implementation.

At the same time, adequate unit tests should be added to test and verify the changes.