feat: add joinHostPort and splitHostPort network functions

[ricardbejarano]

Fixes #472 in the upstream project (#472).

What

Adds two new template functions to the network category:

• joinHostPort host port — wraps net.JoinHostPort. Combines a host and port into host:port, correctly bracketing IPv6 addresses as [host]:port.
• splitHostPort hostport — wraps net.SplitHostPort. Splits a host:port string into a map[string]string with keys host and port, handling IPv6 bracket notation transparently.

Why

urlJoin/urlParse exist for full URLs, but there was no idiomatic way to construct or parse bare host:port endpoints, which is a common need when templating network configurations (proxies, listeners, upstream addresses). IPv6 addresses require bracket notation that is easy to get wrong manually.

Examples

{{ joinHostPort "192.0.2.1" "80" }}       -> 192.0.2.1:80
{{ joinHostPort "2001:db8::1" "80" }}     -> [2001:db8::1]:80

{{ with splitHostPort "192.0.2.1:80" }}host={{ .host }} port={{ .port }}{{ end }}
-> host=192.0.2.1 port=80

{{ with splitHostPort "[2001:db8::1]:80" }}host={{ .host }} port={{ .port }}{{ end }}
-> host=2001:db8::1 port=80

Changes

• network.go: implement joinHostPort and splitHostPort
• network_test.go: unit tests covering IPv4, IPv6, and hostname inputs
• functions.go: register both functions in the function map and the non-sandbox list
• docs/network.md: documentation for both functions
• CHANGELOG.md: entry under unreleased 3.4.0