Support external TLS offloaders in MaxGUI

Modern network infrastructures often implement centralised TLS offloaders (which operate as reverse proxies) for web-based services (eg., to facilitate centralised certificate management). These reverse proxies typically inform the server of the original client protocol using the X-Forwarded-Proto HTTP header, which may have the value of "http" or "https".

Currently, when MaxScale has no certificate configured, it only prints a short message over HTTP saying it needs HTTPS. The following is required to properly support TLS offloaders:

• When running without a certificate, MaxScale should check the presence of X-Forwarded-Proto HTTP header. If it is present and its value is "https", MaxScale should consider the client connection to be secure and should load the full GUI.

• To ensure only a trusted TLS offloader is allowed to set this header, MaxScale may implement a new configuration parameter with a name like "admin_trusted_proxy", which should take a (comma-separated) list of IP addresses, possibly in CIDR notation.

In addition to the mentioned header, a TLS offloader would usually also send the X-Forwarded-For HTTP header, containing the IP address of the actual client; this may be used to support TLS offloaders in a possible implementation of server-side ACL for using the MaxGUI.