[feature request] Be able to specify HTTP auth parameters when creating subscriptions

Currently there is no way to have open-source Kapacitor embed HTTP auth credentials in a subscription. This can be useful for users who are routing subscriptions through a proxy, and enforcing authentication at the proxy layer (transparent to Kapacitor). Similar to the `kapacitor-hostname` and `kapacitor-port` configuration options available to `influxdb` blocks, a `http-auth-username` / `http-auth-password` option would be needed to inject user credentials when creating subscriptions.

For example, the configuration:

```
[[influxdb]]
http-auth-username = "myuser"
http-auth-password = "mypassword"
```

Would generate subscriptions with a target similar to:

```
http://myuser:mypassword@myhost.example.com:9092
```

Trace how influxdb block configuration is parsed and how subscriptions are created, starting with the existing kapacitor-hostname and kapacitor-port options. Check the subscription target construction and related tests, then verify that configured HTTP credentials produce the requested authenticated target without affecting configurations that omit them.

Written by the indexing model from the issue text.