Publish pre-existing package

We would like the ability to publish a pre-existing package. In order to enable "rollback", we archive the package that was deployed with a timestamp of its deployment. We'd use

```
func pack
```

to package the applications, and then run something like

```
func azure functionapp publish {appName} myPackage.zip
```

Start by tracing the existing `func azure functionapp publish` command and the `func pack` workflow mentioned in the issue. Done means the publish command accepts an existing package such as `myPackage.zip` and supports the stated rollback use case of publishing an archived deployment package.

Written by the indexing model from the issue text.