Add Flutter example for speaker identification (#894)

sherpa-onnx/flutter/example/macos/Runner/Configs/AppInfo.xcconfig

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+// Application-level settings for the Runner target.
+//
+// This may be replaced with something auto-generated from metadata (e.g., pubspec.yaml) in the
+// future. If not, the values below would default to using the project name when this becomes a
+// 'flutter create' template.
+
+// The application's name. By default this is also the title of the Flutter window.
+PRODUCT_NAME = sherpa_onnx
+
+// The application's bundle identifier
+PRODUCT_BUNDLE_IDENTIFIER = com.k2fsa.sherpa.onnx
+
+// The copyright displayed in application information
+PRODUCT_COPYRIGHT = Copyright © 2024 com.k2fsa.sherpa.onnx. All rights reserved.

sherpa-onnx/flutter/example/macos/Runner/Configs/Debug.xcconfig

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+#include "../../Flutter/Flutter-Debug.xcconfig"
+#include "Warnings.xcconfig"

sherpa-onnx/flutter/example/macos/Runner/Configs/Release.xcconfig

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+#include "../../Flutter/Flutter-Release.xcconfig"
+#include "Warnings.xcconfig"

sherpa-onnx/flutter/example/macos/Runner/Configs/Warnings.xcconfig

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+WARNING_CFLAGS = -Wall -Wconditional-uninitialized -Wnullable-to-nonnull-conversion -Wmissing-method-return-type -Woverlength-strings
+GCC_WARN_UNDECLARED_SELECTOR = YES
+CLANG_UNDEFINED_BEHAVIOR_SANITIZER_NULLABILITY = YES
+CLANG_WARN_UNGUARDED_AVAILABILITY = YES_AGGRESSIVE
+CLANG_WARN__DUPLICATE_METHOD_MATCH = YES
+CLANG_WARN_PRAGMA_PACK = YES
+CLANG_WARN_STRICT_PROTOTYPES = YES
+CLANG_WARN_COMMA = YES
+GCC_WARN_STRICT_SELECTOR_MATCH = YES
+CLANG_WARN_OBJC_REPEATED_USE_OF_WEAK = YES
+CLANG_WARN_OBJC_IMPLICIT_RETAIN_SELF = YES
+GCC_WARN_SHADOW = YES
+CLANG_WARN_UNREACHABLE_CODE = YES