[API Compat] Adding a new public member to an unsealed public class is not detected as a breaking change

[bitbonk]

Is your feature request related to a problem? Please describe.

When adding a new public (or protected) member to an existing public class (or struct), these changes are not detected as source breaking changes by the baseline package validator. Such changes are not source compatible because consuming code may have inherited from that type and may have already added a member with the same name and signature and this would result in a CS0108 compiler warning that the inherited member is hidden.

Describe the solution you'd like

Introduce a new validation check that fails when a new public or protected member is added to a unsealed class or struct.