Support XCFramework Signatures

[Starting on May 1st](https://developer.apple.com/news/?id=3d8a9yyh), Apple will require that the frameworks on this [list](https://developer.apple.com/support/third-party-SDK-requirements/) are signed (if they are distributed as binary).

I didn't find any documentation showing how this validation will be done by Apple, but I did a demo with Xcode + Lottie 4.4.1 (which is already being distributed with signature and privacy manifest).

There is a new directory `Signatures` inside the `.ipa` that Xcode 15 creates:

Seems likely that Apple will use this `Signatures` directory to validate frameworks in AppStoreConnect. If that's true, Bazel will need to support this directory to avoid blocking the upload of apps.

I still haven't found how to generate this `.signature` file. I just know that it is a plist containing certificate data.

My demo is available here: https://github.com/andre-alves/privacy_demo

Use the privacy_demo and the Xcode 15-generated .ipa shown in the issue as references, and inspect how the Signatures directory and .signature plist appear in the archive. Determine the required generation and packaging behavior, then verify that a Bazel-built binary distribution contains the expected signatures and can pass App Store Connect validation.

Written by the indexing model from the issue text.