Add a --skip-signing option when building macos / ios apps

### What is the problem or limitation you are having?

I'm looking to embed the `.app` produced by `briefcase build` in another `.app` bundle. As such I would like the flexibility to postpone the signing of the build products to later, when I actually do the embedding.

### Describe the solution you'd like

`briefcase build --skip-signing` would do everything but sign the code.

### Describe alternatives you've considered

I can remove the signing information with `codesign --remove-signature` and then re-sign later when assembling the final `.app` bundle.

### Additional context

I've already implemented this for my local use with a `pyproject.toml` setting:
```python
skip_signing = true
```
but i think it's probably better as a CLI option.

Start by locating the macOS and iOS build signing logic and the existing pyproject.toml setting mentioned in the issue. Trace how build options are exposed on the CLI, then verify that --skip-signing leaves the build products unsigned while normal builds retain their current behavior.

Written by the indexing model from the issue text.