sharing SerializationInfo for corert and coreclr (#18102)

sharing SerializationInfo for corert and coreclr (#18102)

* String to string in SerializationInfo

* Object to object in SerializationInfo

* Reducing diff between coreclr and corefx by renaming some variables.

* Reducing diff for SerializationInfo by simplifying code

* More cleanup
- Removing unused fields and making internals private.
- Removing unused namespaces
- Merge some comments

* Fixing up commets and removing object from object.ReferenceEquals(..)

* Minor fixups

* Moving to shared

* Applying PR feedbacks

* using IsRuntimeImplemented helper method for type

* using the CoreRT version and keeping ctor with extra argument