Fixed ExportFile not always truncating the overwritten file (fixed #61)

• BREAKING CHANGE: CheckPermission returns bool instead of Permission (see yasirkula/UnityAndroidRuntimePermissions#14)
• BREAKING CHANGE: Replaced RequestPermission with RequestPermissionAsync (see yasirkula/UnityNativeGallery#343)
• Updated Unity version to 2021.3.41f1 (simplified codebase accordingly)
• iOS frameworks are now added properly instead of changing OTHER_LDFLAGS
• Fixed Xcode compiler warnings

• Added native PermissionFreeMode flag on Android to skip permission dialog (not recommended)
• Added some more native Android logs to help debug certain scenarios

Fixed rare "No such proxy method" error for native callback functions

• If no allowedFileTypes is specified, all files can be selected
• ConvertExtensionToFileType throws an exception for "/" input (very common mistake, see FAQ)
• Fixed permission not being asked in an edge case on Android (it's strongly recommended to use RequestPermissionAsync)

Potential fix for IllegalStateException on Android

• Added async variants of RequestPermission function that don't freeze the app unnecessarily
• Fixed a native IllegalStateException

• Fixed Denied permission issue on Android 13 and later
• Added params keyword to PickFile.allowedFileTypes parameter