Add explicit host entries to container configuration

[mazdak]

Problem

While building a Docker Compose-like plugin for container and validating it against our real Docker Compose workload, we hit a core limitation: there was no way for callers to ask the runtime to append explicit entries to a container's /etc/hosts.

That showed up most clearly with Compose extra_hosts, especially the common host.docker.internal pattern. The plugin could parse those mappings, but there was no core field to carry them into the sandbox, so the runtime always generated only the default localhost/container-name entries.

In practice, containers that depended on host aliases still failed name resolution even though the compose file specified them.

Why this belongs in core

This is not something the plugin can fake safely. /etc/hosts is generated in the sandbox layer, so callers need a first-class way to provide additional host entries to the runtime.

What this changes

• add ContainerConfiguration.HostEntry
• add ContainerConfiguration.hosts
• preserve those entries through configuration encoding/decoding
• extend sandbox host generation to append caller-provided host entries after the default localhost and primary hostname entries
• add focused tests for round-tripping and host resolution behavior

Validation

• swift test --filter ContainerConfigurationTest
• swift test --filter SandboxServiceHostsTest

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