Protobuf contract must not contain map<string, string> types

[mhoeger]

In our protobuf contract, "map" type values should never be a native type (if it can be a null value), because it causes a Buffer overrun in JavaScript (and other languages).

Azure/azure-functions-language-worker-protobuf#21

This issue is to track a potential major version change in the protobuf contract to change all map<string, string> to map<string, NullableString> (or other nullable type

cc: @pragnagopa

[fabiocav]

After talking to @mhoeger, removing the breaking flag as we can make this change in a non-breaking way