Support Custom Handlers

## func init

Add `custom` as a worker runtime option. (@pragnagopa is "custom" a good name?)

```
func init --worker-runtime custom
```

If `custom` is selected, add empty `customHandler` section to host.json.

## func new

Allow user to select from a set of templates. When selected, only create function folder and function.json. (@soninaren Should we add custom handler templates or should we use the JS ones and remove index.js for now?)

## func azure functionapp publish

Currently, we need to publish using `func azure functionapp publish $functionAppName --no-build --force`. Allow publishing without the extra flags.

```
func azure functionapp publish $functionAppName
```

Start with the `func init`, `func new`, and `func azure functionapp publish` command entry points; the issue does not name implementation files or tests. Clarify the custom handler template choices and publishing behavior, then verify that initialization updates host.json, creation produces the requested files, and publishing works without extra flags.

Written by the indexing model from the issue text.