Enable SPM support for Ext and configure pipelines for extensions target testing (#3)

* initial ext files

* add a workable version of spm for extensions

* update

* update pipeline

* update pipelines

* fix dev/release pipelines for extensions pod

* fix dev pipelines for extensions pod

* fix release pipelines for extensions pod

* fix dev  pipelines for extensions pod

* adding empty include folder for configuring extensions target path

* test

* test

* test

* revert pipeline changes

* revert gitignore changes

* add ext pod binary target for release pipeline

* add pipeline for extensions

* update

* update package.swift

* update latest from branch

* update

* fix

* fix

* update

* try dummy empty file

* test

* update package.swift to use fatalerror

* syntax

* try gitignore revert

* update gitignore

* add -list

* onnxruntime-Package

* update using onnxruntime-Package

* update dev pipeline

* fix dev pipelines

* syntax

* pull extensions/

* update pipelines again

* syntax

* variables

* fix

* fix -r

* update Package.swift

* update Package.swift

* minor update

* address pr comments

* minor updates

* fix

* refine messages

* syntax

* syntax again

* address pr comments partial

* address pr comments

* add .h header file and notes

* minor updates

* syncing objc source files and add code to register custom ops using function pointer

* format

* move to the header for function doc

---------

Co-authored-by: rachguo <rachguo@rachguos-Mini.attlocal.net>
Co-authored-by: rachguo <rachguo@rachguos-Mac-mini.local>

extensions/include/OrtExt.h

@@ -0,0 +1,17 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+#import <Foundation/Foundation.h>
+
+@interface OrtExt : NSObject
+
+typedef struct OrtStatus* (*ORTCAPIRegisterCustomOpsFnPtr)(struct OrtSessionOptions* /*options*/,
+                                                           const struct OrtApiBase* /*api*/);
+
+// Note: This returns the address of `RegisterCustomOps` function. At swift
+// level, user can call this function to get the RegisterCustomOpsFnPtr
+// and use the function pointer to register custom ops. See
+// SwiftOnnxRuntimeExtensionsTests.swift for an example usage.
++ (nonnull ORTCAPIRegisterCustomOpsFnPtr)getRegisterCustomOpsFunctionPointer;
+
+@end