Allow adding multiple CIDR-based announce-ip of wsproxy coordinator in the webserver

In some customer sites, there are two groups of users who connects from different networks (e.g., internal & public) and the wsproxy address must be differentiated for each group of users.

We need to provide an option to configure additional CIDR-based wsproxy address announcements.

Currently, `[service].wsproxy.url` is configured in `webserver.conf` and this value is propagated and rendered to `config.toml` and `config_ini.toml` which are fetched by the WebUI.

Let's keep the existing configuration to work as "0.0.0.0/0" fallback option and allow addition of CIDR prefix based mappings:

## For wsproxy v1 (used in all-in-one dev setup)

**Before: (webserver.conf)**

```toml
[service]
wsproxy.url = "example.com:5050"
```

**After: (webserver.conf)**

```toml
[service]
wsproxy.url = "example.com:5050" # serves as "0.0.0.0/0" fallback

[service.wsproxy.prefixed-urls]
"10.123.0.0/16" = "example.internal:5051"
"192.168.0.0/24" = "example2.internal:5051"
```

This will be parsed like:

```python
{'service': {'wsproxy': {'prefixed-urls': {'10.123.0.0/16': 'example.internal:5051',
'192.168.0.0/24': 'example2.internal:5051'},
'url': 'example.com:5050'}}}
```

## For wsproxy v2 (used in production and large-scale deployments)

We need to extend the `scaling_groups` table definition to hold a mapping of CIDR-prefixes to wsproxy URLs. This per-resource-group setting is used by the manager's `start_service` API which is usually invoked via the webserver. Fortunately the webserver already provides the actual client IP address via the `X-Forwarded-For` header, so that the manager could reply with the appropriate mapping.

JIRA Issue: BA-150