Support for hostname-based proxy server configuration in KumoMTA's egress sources

[pankajrathi95]

Summary

This PR adds support for hostname-based proxy server configuration in KumoMTA's egress sources. Previously, ha_proxy_server and socks5_proxy_server only accepted SocketAddr format (e.g., "127.0.0.1:8080"). Now they support both SocketAddr and hostname formats, with hostnames being resolved to IP addresses at runtime when connections are established.

Code Changes

crates/kumod/src/egress_source.rs:

• Added SocketAddrOrHostname enum to support both SocketAddr and hostname formats
• Updated EgressSource struct to use SocketAddrOrHostname for proxy server fields
• Enhanced resolve_proxy_protocol method to handle hostname resolution asynchronously
• Added comprehensive test coverage for the new functionality

Documentation updates

• docs/reference/kumo/make_egress_source/ha_proxy_server.md: Updated to document hostname support with inline version indicator
• docs/reference/kumo/make_egress_source/socks5_proxy_server.md: Updated to document hostname support with inline version indicator
• docs/userguide/operation/proxy.md: Added examples showing both SocketAddr and hostname usage