`DomainsResponse` is not always parsed correctly

The Domain model on FluxC is expecting the currentUserCannotAddEmailReason property to be a string, when in fact it is an object instead.

Here's an example of what the data in that property might look like:

"current_user_cannot_add_email_reason": {
	"errors": {
		"other-user-owns-subscription": [
			"Only the owner of the domain subscription for atestdesignportfolio.com can buy email services for that domain"
		]
	},
	"error_data": []
}

This causes the Domains dashboard to be empty upon opening after it silently fails with the following error:

2021-10-20 20:01:06.764 6626-6626/org.wordpress.android.beta E/WordPress-API: Volley error on https://public-api.wordpress.com/rest/v1.1/sites/186366855/domains/?locale=en_US
    com.android.volley.ParseError: com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected a string but was BEGIN_OBJECT at line 1 column 1854 path $.domains[1].current_user_cannot_add_email_reason
        at org.wordpress.android.fluxc.network.rest.GsonRequest.parseNetworkResponse(GsonRequest.java:98)
        at com.android.volley.NetworkDispatcher.processRequest(NetworkDispatcher.java:143)
        at com.android.volley.NetworkDispatcher.processRequest(NetworkDispatcher.java:111)
        at com.android.volley.NetworkDispatcher.run(NetworkDispatcher.java:90)
     Caused by: com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected a string but was BEGIN_OBJECT at line 1 column 1854 path $.domains[1].current_user_cannot_add_email_reason
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:226)
        at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:41)
        at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:82)
        at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:61)
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:131)
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:222)
        at com.google.gson.Gson.fromJson(Gson.java:927)
        at com.google.gson.Gson.fromJson(Gson.java:892)
        at com.google.gson.Gson.fromJson(Gson.java:841)
        at com.google.gson.Gson.fromJson(Gson.java:813)
        at org.wordpress.android.fluxc.network.rest.GsonRequest.parseNetworkResponse(GsonRequest.java:92)
        at com.android.volley.NetworkDispatcher.processRequest(NetworkDispatcher.java:143) 
        at com.android.volley.NetworkDispatcher.processRequest(NetworkDispatcher.java:111) 
        at com.android.volley.NetworkDispatcher.run(NetworkDispatcher.java:90) 
     Caused by: java.lang.IllegalStateException: Expected a string but was BEGIN_OBJECT at line 1 column 1854 path $.domains[1].current_user_cannot_add_email_reason
        at com.google.gson.stream.JsonReader.nextString(JsonReader.java:825)
        at com.google.gson.internal.bind.TypeAdapters$16.read(TypeAdapters.java:402)
        at com.google.gson.internal.bind.TypeAdapters$16.read(TypeAdapters.java:390)
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:131)
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:222)
        at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:41) 
        at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:82) 
        at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:61) 
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:131) 
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:222) 
        at com.google.gson.Gson.fromJson(Gson.java:927) 
        at com.google.gson.Gson.fromJson(Gson.java:892) 
        at com.google.gson.Gson.fromJson(Gson.java:841) 
        at com.google.gson.Gson.fromJson(Gson.java:813) 
        at org.wordpress.android.fluxc.network.rest.GsonRequest.parseNetworkResponse(GsonRequest.java:92) 
        at com.android.volley.NetworkDispatcher.processRequest(NetworkDispatcher.java:143) 
        at com.android.volley.NetworkDispatcher.processRequest(NetworkDispatcher.java:111) 
        at com.android.volley.NetworkDispatcher.run(NetworkDispatcher.java:90) 

We should probably remove all the properties that we don't know for sure we are going to need.

Start with the Domain model in fluxc/src/main/java/org/wordpress/android/fluxc/network/rest/wpcom/site/DomainsResponse.kt, especially currentUserCannotAddEmailReason, and compare it with the example response in this issue. Confirm parsing the object no longer produces the shown Gson error and that the Domains dashboard is not empty when this response is present.