prost-build: derive Clone on builders

This could be useful to e.g. initialize common fields for multiple
messages, then clone the builder to set the fields that are different.

prost-build/src/code_generator.rs

@@ -770,7 +770,7 @@ impl<'a> CodeGenerator<'a> {
             "/// Builder for [`{struct_name}`](super::{struct_name})\n"
         ));
         self.push_indent();
-        self.buf.push_str("#[derive(Default)]\n");
+        self.buf.push_str("#[derive(Clone, Default)]\n");
         self.push_indent();
         self.buf.push_str("pub struct ");
         self.buf.push_str(builder_name);