feat: Make generated ProtoMessage and ProtoEnum classes final (#2349)

Author: brianquinlan

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

@@ -17,7 +17,7 @@ limitations under the License.
 {{#Codec.DocLines}}
 {{{.}}}
 {{/Codec.DocLines}}
-class {{Codec.Name}} extends ProtoEnum {
+final class {{Codec.Name}} extends ProtoEnum {
   {{#Values}}
   {{#Codec.DocLines}}
   {{{.}}}

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

@@ -18,7 +18,7 @@ limitations under the License.
 {{#Codec.DocLines}}
 {{{.}}}
 {{/Codec.DocLines}}
-class {{Codec.Name}} extends ProtoMessage {
+final class {{Codec.Name}} extends ProtoMessage {
   static const String fullyQualifiedName = '{{Codec.QualifiedName}}';
 
   {{#Fields}}