[ObjC] Start of the internal api for `GPBUnknownFields`

Add in the support for pushing the data back onto a `GPBMessage`.

PiperOrigin-RevId: 649125923
diff --git a/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj b/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
index a01147a..63516bf 100644
--- a/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
+++ b/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
@@ -75,6 +75,7 @@
 		F43ADD472C2F387A005312E5 /* GPBCompileTest25.m in Sources */ = {isa = PBXBuildFile; fileRef = F43ADD462C2F387A005312E5 /* GPBCompileTest25.m */; };
 		F43ADD522C333E58005312E5 /* GPBUnknownFields+Additions.swift in Sources */ = {isa = PBXBuildFile; fileRef = F43ADD512C333E58005312E5 /* GPBUnknownFields+Additions.swift */; };
 		F43ADD582C345D0D005312E5 /* GPBUnknownField+Additions.swift in Sources */ = {isa = PBXBuildFile; fileRef = F43ADD572C345D0D005312E5 /* GPBUnknownField+Additions.swift */; };
+		F43ADD5E2C35A6E1005312E5 /* GPBUnknownFields_PackagePrivate.h in Headers */ = {isa = PBXBuildFile; fileRef = F43ADD5D2C35A6E1005312E5 /* GPBUnknownFields_PackagePrivate.h */; };
 		F43C88D0191D77FC009E917D /* text_format_unittest_data.txt in Resources */ = {isa = PBXBuildFile; fileRef = F43C88CF191D77FC009E917D /* text_format_unittest_data.txt */; };
 		F4487C6A1A9F8F8100531423 /* Foundation.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 1D30AB110D05D00D00671497 /* Foundation.framework */; };
 		F4487C6F1A9F8FFF00531423 /* GPBProtocolBuffers.m in Sources */ = {isa = PBXBuildFile; fileRef = 8BCF338814ED799900BC5317 /* GPBProtocolBuffers.m */; };
@@ -218,6 +219,7 @@
 		F43ADD462C2F387A005312E5 /* GPBCompileTest25.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GPBCompileTest25.m; sourceTree = "<group>"; };
 		F43ADD512C333E58005312E5 /* GPBUnknownFields+Additions.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = "GPBUnknownFields+Additions.swift"; sourceTree = "<group>"; };
 		F43ADD572C345D0D005312E5 /* GPBUnknownField+Additions.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = "GPBUnknownField+Additions.swift"; sourceTree = "<group>"; };
+		F43ADD5D2C35A6E1005312E5 /* GPBUnknownFields_PackagePrivate.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = GPBUnknownFields_PackagePrivate.h; sourceTree = "<group>"; };
 		F43C88CF191D77FC009E917D /* text_format_unittest_data.txt */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text; path = text_format_unittest_data.txt; sourceTree = "<group>"; };
 		F4411BE81AF1301700324B4A /* GPBArray_PackagePrivate.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = GPBArray_PackagePrivate.h; sourceTree = "<group>"; };
 		F4487C6E1A9F8F8100531423 /* libTestSingleSourceBuild.a */ = {isa = PBXFileReference; explicitFileType = archive.ar; includeInIndex = 0; path = libTestSingleSourceBuild.a; sourceTree = BUILT_PRODUCTS_DIR; };
@@ -427,6 +429,7 @@
 				7461B4AE0F94F99000A0C422 /* GPBUnknownField.h */,
 				7461B4AF0F94F99000A0C422 /* GPBUnknownField.m */,
 				F43ADD572C345D0D005312E5 /* GPBUnknownField+Additions.swift */,
+				F43ADD5D2C35A6E1005312E5 /* GPBUnknownFields_PackagePrivate.h */,
 				F43ADD372C2F2D06005312E5 /* GPBUnknownFields.h */,
 				F43ADD362C2F2D06005312E5 /* GPBUnknownFields.m */,
 				F43ADD512C333E58005312E5 /* GPBUnknownFields+Additions.swift */,
@@ -586,6 +589,7 @@
 			isa = PBXHeadersBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				F43ADD5E2C35A6E1005312E5 /* GPBUnknownFields_PackagePrivate.h in Headers */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};