Expose numerical site ID trough the API

[ridercz]

Currently there is no way how to get the numerical site ID used by the rest of IIS administration tools. I think the API should expose this information in addition to your internal string ID, because it's basically the only way how to reliably correlate sites across different administration tools/APIs.

[jimmyca15]

The site resource already has a field with the name id. We must use a name that does not cause confusion with the API id of the site.

Options discussed:

• config_id
• apphost_id
• config_key
• site_number
• config_number
• site_index

[jimmyca15]

Implemented in 7b378a2