[ios][4.3-beta] expose explict module for c & cpp

[pbk20191]

• I confirm this contribution is made under the Esoteric Software LLC CLA.

This Pull Request goal is to add ability to access cpp interface directly using same package for swift package manager system.

• standard user can import spine-c based api without c warning
• dedicated user can import cpp interface with explict import and enabling cpp interp
• objective-c user can import cpp using objective-c++ without issue

there is 3 possibility in user side.

1. swift with objective-c/C interp mode (default)
2. swift with Cpp interp mode (not default)
3. swift with objective-c++ (this is different from 2 for cpp only case)

• cpp interface imported by swift interp mode can not be passed back to objective-c/objective-c++

swift with cpp interp mode user

import SpineC.cpp

func dummy() {
    var renderer = spine.SkeletonRenderer()
    
}
import SpineC

func dummyBridged(_ ptr:spine_skeleton_renderer) {
   var renderer = UnsafeMutablePointer<spine.SkeletonRenderer>(OpaquePointer(ptr))

}

swift package user can enable their cpp interp mode on their own

        .target(
            name: "FOO2",
            dependencies: [
                .product(name: "SpineC", package: "spine-runtimes")
            ],
            path: "spine-ios/Sources/Foo2",
            swiftSettings: [
                .interoperabilityMode(.Cxx)
            ]
        ),

Xcode project setting coresponds to this is SWIFT_OBJC_INTEROP_MODE

objective-c++ user

/// some .mm file not .m file
#include <spine/spine.h> // this for standard header include, standard for objective-c++
@import SpineC.cpp // this is for clang module import rule, not standard for objective-c++
#import <Foundation/Foundation.h>

void fooo(void) {
    spine::SpineExtension::getInstance();
    
    NSString* foo = @"";
}

if the user import cpp header from objective-c file, user will get error named

Module 'SpineC.cpp' requires feature 'cplusplus11'

if the user tries to import SpineC.cpp from swift file, without cpp interp mode enable from their side

Module 'SpineC.cpp' requires feature 'cplusplus11'
No such module 'SpineC.cpp'

My project highly depends on mixed objective-c/c++ & swift. So ability to directly access spine-cpp interface is really appreciated. ( customized spine::TextureLoader, cpp callback from spine::AnimationState ... etc)