Introduce ComponentConfig-style manager configuration #1032

Summary

Introduce a configuration-file / ComponentConfig-style configuration model for the metal-operator manager, reducing reliance on additional flag.Var registrations.

The manager already exposes more than 40 flags. Future settings—such as configuring the discovery-token signing-secret behavior introduced in #750—should have a scalable, structured configuration path instead of continuing to expand the CLI flag surface.

Rationale

A typed configuration structure can improve discoverability, validation, defaults, and long-term maintainability while preserving a clear migration story for existing deployments.

Affected areas

• Manager startup and CLI flag registration in cmd/main.go
• Controller and registry configuration currently supplied through manager flags
• Configuration documentation and deployment manifests
• Tests for configuration parsing, defaults, and validation

Required changes

• Define a versioned, typed ComponentConfig-style API or configuration-file schema for manager settings.
• Support loading and validating configuration from a file at startup.
• Establish defaults and precedence rules between configuration-file values and existing CLI flags.
• Migrate or provide a documented path to migrate existing manager settings without introducing new flag.Var calls for future options.
• Document the configuration format, validation failures, and migration/compatibility behavior.

Acceptance criteria

• The manager can load a typed configuration file and rejects invalid configuration with actionable errors.
• Existing deployments using current flags retain a documented compatibility path.
• Configuration precedence and defaults are explicitly tested and documented.
• At least one currently flag-based manager setting is demonstrated through the new configuration mechanism.
• Documentation describes how future components/settings should be added without increasing manager flag proliferation.

Context

Requested by @stefanhipfel during review of #750.

• PR: Secure discovery boot data with jwt signed tokens #750
• Review discussion: Secure discovery boot data with jwt signed tokens #750 (comment)