feat: add internal host detection support

[ruben-rodriguez]

Adds internal host detection support (reverse DNS lookup).

Fixes #377

After SAML auth, portal sends in the config a block like the following:

<internal-host-detection>
      <ip-address></ip-address>
      <host></host>
      <ipv6-address/>
      <ipv6-host/>
</internal-host-detection>

It is meant to be resolved by the client in order to present appropiate gateway list to the user, internal or external gateways, depending on the success of the DNS lookup:

<gateways>
        <internal>
          <list>
                <entry name="xxx.xxx.xxx.xxx">
                    <priority-rule>
                        <entry name="Any">
                            <priority>1</priority>
                        </entry>
                    </priority-rule>
                    <priority>1</priority>
                    <description>vpn_gateway</description>
                </entry>
            </list>
        </internal>
        <cutoff-time>5</cutoff-time>
        <external>
            <list>
                <entry name="xxx.xxx.xxx.xxx">
                    <priority-rule>
                        <entry name="Any">
                            <priority>1</priority>
                        </entry>
                    </priority-rule>
                    <priority>1</priority>
                    <description>vpn_gateway</description>
                </entry>
            </list>
        </external>
    </gateways>

Sample output when DNS lookup fails (external gateways list is provided):

[2024-07-01T21:16:04Z INFO  gpclient::cli] gpclient started: 2.3.3 (2024-07-01)
[2024-07-01T21:16:04Z INFO  gpapi::portal::prelogin] Portal prelogin with user_agent: PAN GlobalProtect
[2024-07-01T21:16:04Z INFO  gpauth::cli] gpauth started: 2.3.3 (2024-07-01)
[2024-07-01T21:16:04Z INFO  gpauth::cli] Please continue the authentication process in the default browser
[2024-07-01T21:16:04Z INFO  gpclient::connect] Waiting for the browser authentication to complete...
[2024-07-01T21:16:04Z INFO  gpclient::connect] Listening authentication data on port 41875
[2024-07-01T21:16:38Z INFO  gpclient::connect] Received the browser authentication data from the socket
[2024-07-01T21:16:38Z INFO  gpapi::auth] Parsing SAML auth data...
[2024-07-01T21:16:38Z INFO  gpapi::portal::config] Portal config, user_agent: PAN GlobalProtect
[2024-07-01T21:16:38Z INFO  gpapi::portal::config] internal-host-detection returned, performing DNS lookup
[2024-07-01T21:16:38Z WARN  gpapi::portal::config] DNS lookup failed for x.x.x.x: failed to lookup address information: Name or service not known
? Which gateway do you want to connect to?
> Germany
  Netherlands
  Hong Kong
  US
  UK

I tested the internal use-case by making the DNS resolution successful in local /etc/hosts file.

Rust is not a programming language I'm used to, and I'm happy to make any modifications that could improve the code.

[yuezk]

Hi @ruben-rodriguez, Thank you so much for implementing this. I will release this feature soon.