[JavaTypeSystem] Add JavaFieldModel.ToString ().

[jpobst]

Context: https://discord.com/channels/732297728826277939/732297837953679412/1151531791069499524

A user reported the following output in their java-resolution-report.log:

The field 'Java.Interop.Tools.JavaTypeSystem.Models.JavaFieldModel' was removed because its name contains a dollar sign.
The class '[Class] com.google.android.libraries.navigation.internal.aac.ad' was removed because the Java base type 'com.google.android.libraries.navigation.internal.aad.ar<com.google.android.libraries.navigation.internal.aac.af<K, V>>' could not be found.

We should be providing the user with the name of the removed field rather than the JavaFieldModel type name. To do this, add an appropriate JavaFieldModel.ToString ().