Add stellar container use to set the default engine - #2647

What

Adds two subcommands for managing a persistent default container engine:

• stellar container use <engine> — writes the chosen engine (docker or apple-container) to the global config.toml under defaults.container_engine.
• stellar container unset — removes that persisted default.

On startup the persisted default is loaded into STELLAR_CONTAINER_ENGINE via set_env_from_config, so it feeds the existing engine resolution without a new code path. An explicit --engine flag or an already-set STELLAR_CONTAINER_ENGINE still takes precedence, and container use warns when the env var is set since it would override the value being saved.

Config plumbing: a container_engine field on Defaults, set_container_engine/unset_container_engine on Config, and write_default_container_engine/unset_default_container_engine on the locator.

Why

Builds on the --engine flag (base branch) so users no longer have to pass --engine on every stellar container invocation. Setting the engine once persists it across commands, matching how other stellar defaults (network, identity) already work.

Known limitations

N/A