feat: Make the Dart class representing the service final (#2178)

Dart classes are, by default, also interfaces. This means that adding a
new method to a class will break any code that implements it.

The most conservative approach is to make the class `final` - we can
relax that restriction and make it `base` later, if it makes sense.

Author: brianquinlan

internal/sidekick/internal/dart/templates/lib/service.mustache

@@ -17,7 +17,7 @@ limitations under the License.
 {{#Codec.DocLines}}
 {{{.}}}
 {{/Codec.DocLines}}
-class {{Codec.Name}} {
+final class {{Codec.Name}} {
   static const String _host = '{{DefaultHost}}';
 
   final ServiceClient _client;