WebFunctionsManager#CreateOrUpdate does not set Files property in response

# Investigative information

Missing content of property `Files` in return value when calling `WebFunctionsManager#CreateOrUpdate`.

Following class contains a field named `Files`, which is not filled by the following call chain:
https://github.com/Azure/azure-functions-host/blob/4556a24d806403e9dc034ce79dcfce7b480cf6dc/src/WebJobs.Script.WebHost/Models/FunctionMetadataResponse.cs#L69

Call chain:

https://github.com/Azure/azure-functions-host/blob/4556a24d806403e9dc034ce79dcfce7b480cf6dc/src/WebJobs.Script.WebHost/Controllers/FunctionsController.cs#L68

https://github.com/Azure/azure-functions-host/blob/4556a24d806403e9dc034ce79dcfce7b480cf6dc/src/WebJobs.Script.WebHost/Management/WebFunctionsManager.cs#L87

https://github.com/Azure/azure-functions-host/blob/4556a24d806403e9dc034ce79dcfce7b480cf6dc/src/WebJobs.Script.WebHost/Management/WebFunctionsManager.cs#L162

https://github.com/Azure/azure-functions-host/blob/4556a24d806403e9dc034ce79dcfce7b480cf6dc/src/WebJobs.Script.WebHost/Extensions/FunctionMetadataExtensions.cs#L23
#### Repro steps

Provide the steps required to reproduce the problem:

1. Calling the REST endpoint using a valid payload to create a Function
2. Await HTTP response and investigate the JSON payload. The `files` property is empty.

#### Expected behavior

- `files` property should be filled with the passed values of the POST payload.

#### Actual behavior

- `files` property is an empty JSON object

#### Known workarounds

none

#### Related information

Provide any related information

* Programming language used: Go (azure-sdk-for-go)

Source

```go
appsClient := web.NewAppsClientWithBaseURI(resourceEndpoint, subscriptionID)
resp, err := client.GetFunction(ctx, resourceGroup, functionAppName, functionName)
resp.Files // should be a filled map, but it's empty
```

Start with FunctionMetadataResponse.cs and trace the listed path through FunctionsController.cs, WebFunctionsManager.cs, and FunctionMetadataExtensions.cs. Reproduce the CreateOrUpdate REST request and inspect its JSON response; done means the files property contains the values supplied in the POST payload rather than an empty object.

Written by the indexing model from the issue text.