[custom provider] allow domain name for VPN_ENDPOINT_IP

[vdrover]

What's the feature 🧐

Custom VPN providers like PureVPN do not provide an IP address in their custom wireguard. Rather, a domain/subdomain is listed and gluetun does not accept this as a valid VPN_ENDPOINT_IP parameter.

I traced the domain to an IP address, and used that instead. However, if someone changes the DNS records for the domain, my connection will break.

Thus, allowing a domain for the VPN_ENDPOINT_IP parameter (that is then resolved to an IP dynamically) would be useful for PureVPN and perhaps other custom VPN providers.

Extra information and references

No response

[qdm12]

Duplicate of #1496