Be able to chose between arm64 and arm64e for MacOS runners

If we want to start testing things on arm64e on GitHub, we should add a configuration option for MacOS runners to chose between an arm64 and arm64e installation.

I suspect for the time being we should default to arm64, as many things will not be compiling to or not be distributed as arm64e just yet.

We already have `architecture` in the docs, so I suspect we can just add `arm64e` there:

```yaml
- uses: dart-lang/setup-dart@7654d458321ee25acccccfdb86cd48bd95768ff1
with:
sdk: dev
architecture: arm64e
```

Context:

* https://github.com/dart-lang/sdk/issues/63420

Start with the existing `architecture` option in the documentation and follow how it selects the macOS runner installation. Add support for choosing `arm64e` while keeping `arm64` as the default, then verify the documented configuration and relevant installation behavior.

Written by the indexing model from the issue text.