Consider Decoupling the Submit-AzureSdkForNetPr Task into Smaller SubTasks

In https://github.com/microsoft/typespec/blob/main/packages/http-client-csharp/eng/pipeline/publish.yml, we run the `login-to-github.yml` template job to authenticate before we run the Submit-AzureSdkForNetPr.ps1 script. The authenticated token is only valid for a few minutes, and the regen job usually takes at most an hour. Rather than having to request a new token inside of the script itself before we call `git push` we should consider splitting the script up into multiple smaller tasks:

1. Regenerate libraries
2. Commit changes
3. Call auth script to fetch token
4. Call `git push`