Worker Indexing - support custom handlers

Allow custom handlers to return functions metadata.

Proposal:

Add a `functionsMetadataRoute` (or something) to the custom handlers config in host.json:

```json
{
"version": "2.0",
"customHandler": {
"description": {
"defaultExecutablePath": "./Handler",
"functionsMetadataRoute": "/functionsmetadata"
}
}
}
```

If the property is specified, instead of indexing functions from the filesystem, make an HTTP GET request to the specified route on the custom handler to retrieve metadata.

@surgupta-msft @fabiocav any thoughts?

Start by locating the custom-handler configuration and the existing filesystem-based function indexing path. Then trace how a configured GET route could return functions metadata, and define completion as successfully indexing functions from that response instead of the filesystem; no file or test is named in the issue.

Written by the indexing model from the issue text.