Add default custom domain as output in the GitHub action

Today I can only get the `defaultHostname` (meaning the one on the azurestaticapps.net domain) via the `static_web_app_url` output. I would like to get the default custom domain as well.

**Describe the solution you'd like**
An output from the github action providing the default customDomain in addition to the `defaultHostname`,

- `[stepid].outputs.static_web_app_defaultDomain` // (www.mydomain.com)
- `[stepid].outputs.static_web_app_url` // (rock-banana12345.1.azurestaticapps.net)

**Describe alternatives you've considered**

I'm using the azure CLI to get it as a separate github action step:
```bash
$ az staticwebapp show -n myApp --query "customDomains[0]" -o tsv
```