install: Add a config knob for container sigpolicy

[jbtrystram]

Surface the --enforce-container-sigpolicy flag as a config file option.

Fixes #2112

Assisted-by: OpenCode.ai <Opus 4.6>

[gemini-code-assist]

Code Review

This pull request introduces support for the enforce-container-sigpolicy configuration option, allowing users to enforce container image signature policies via TOML drop-in files. The changes include updating the InstallConfiguration struct, implementing the necessary merging logic, adding unit tests, and updating the relevant documentation. The reviewer suggested refactoring the configuration merging logic to use if let for better readability and to avoid redundant assignments when configuration values are absent.

[gemini-code-assist]

The logic for merging boolean options from the configuration file can be made more explicit to improve readability and avoid potentially redundant assignments.

When a boolean configuration option is not present, the current implementation with unwrap_or(false) assigns false to a variable that is already known to be false inside the if !... block.

Using if let to check for the presence of the config value before assigning makes the intent clearer and avoids this no-op assignment.

        if !config_opts.bootupd_skip_boot_uuid {
            if let Some(skip) = config.bootupd.as_ref().and_then(|b| b.skip_boot_uuid) {
                config_opts.bootupd_skip_boot_uuid = skip;
            }
        }

        if config_opts.bootloader.is_none() {
            config_opts.bootloader = config.bootloader.clone();
        }

        if !target_opts.enforce_container_sigpolicy {
            if let Some(enforce) = config.enforce_container_sigpolicy {
                target_opts.enforce_container_sigpolicy = enforce;
            }
        }

[cgwalters]

Looks sane. That said I think it'd be helpful for us to

• consolidate our install tests into one thing
• add an install test case for this from a container image source config, not just a synthetic unit test

[cgwalters]

For now just missing cargo fmt