Add experimental_strip_nonfunctional_codegen generation option.

Almost all of the upstream generators seem to be adding this options. Adding it
now to support any future validations that might need it (likely Protobuf
Editions).

Author: thomasvl

Sources/protoc-gen-swift/GeneratorOptions.swift

@@ -53,6 +53,7 @@ class GeneratorOptions {
   let protoToModuleMappings: ProtoFileToModuleMappings
   let visibility: Visibility
   let implementationOnlyImports: Bool
+  let experimentalStripNonfunctionalCodegen: Bool
 
   /// A string snippet to insert for the visibility
   let visibilitySourceSnippet: String
@@ -63,6 +64,7 @@ class GeneratorOptions {
     var visibility: Visibility = .internal
     var swiftProtobufModuleName: String? = nil
     var implementationOnlyImports: Bool = false
+    var experimentalStripNonfunctionalCodegen: Bool = false
 
     for pair in parameter.parsedPairs {
       switch pair.key {
@@ -100,6 +102,15 @@ class GeneratorOptions {
           throw GenerationError.invalidParameterValue(name: pair.key,
                                                       value: pair.value)
         }
+      case "experimental_strip_nonfunctional_codegen":
+        if pair.value.isEmpty {  // Also support option without any value.
+          experimentalStripNonfunctionalCodegen = true
+        } else if let value = Bool(pair.value) {
+          experimentalStripNonfunctionalCodegen = value
+        } else {
+          throw GenerationError.invalidParameterValue(name: pair.key,
+                                                      value: pair.value)
+        }
       default:
         throw GenerationError.unknownParameter(name: pair.key)
       }
@@ -130,6 +141,7 @@ class GeneratorOptions {
     }
 
     self.implementationOnlyImports = implementationOnlyImports
+    self.experimentalStripNonfunctionalCodegen = experimentalStripNonfunctionalCodegen
 
     // ------------------------------------------------------------------------
     // Now do "cross option" validations.