Refactor: introduce sub-module architecture with auto-discovered entity warmer plugins

Problem/Motivation

The main module handled all entity types (nodes, taxonomy terms, custom URLs)
in a single monolithic settings form (MicrowaveSettingsForm) and
a single flattened configuration object (microwave.settings).
Commerce product support was provided by microwave_commerce,
which used a hook_form_alter to inject its configuration section
into the main form by directly manipulating the form array structure. This
created tight coupling between sub-modules and the main module's internal form
structure, making the architecture brittle and hard to extend cleanly.

Adding support for a new entity type required changes to the main module itself.

Steps to reproduce

N/A — this is a refactoring issue with no bug to reproduce.

Proposed resolution

Adopt a sub-module architecture with attribute-based auto-discovered entity
warmer plugins, similar to the approach used in the index_now
module.

• Introduce an EntityWarmerManager that auto-discovers plugins
  using a PHP 8 attribute (#[WarmableEntity]) from
  Plugin/EntityWarmer/ directories across all enabled modules.
• Define an EntityWarmerInterface and an
  AbstractEntityWarmer base class providing shared logic (entity
  querying, date filtering, queue item building).
• Split entity type support into dedicated sub-modules
  (microwave_node, microwave_taxonomy,
  microwave_commerce_product), each containing only a plugin
  class, a config schema, and a default configuration file.
• Rebuild the main settings form dynamically from discovered plugins. Each
  plugin provides its own configuration section — no more form alters.
• Replace the previous per-entity-type Drush commands
  (microwave:process_nodes_urls,
  microwave:process_terms_urls,
  microwave:process_commerce_product_urls) with a single unified
  command microwave:process_entity_urls (alias mpeu)
  that iterates over all discovered entity warmer plugins.
• Keep the queue-based async warming architecture intact; only the entity
  registration and configuration layers are refactored.
• Handle custom URL warming via a dedicated service / queue worker, outside
  of the entity plugin system.

Remaining tasks

• ~~Design and implement EntityWarmerInterface and AbstractEntityWarmer~~ ✓
• ~~Implement EntityWarmerManager with attribute-based discovery~~ ✓
• ~~Create microwave_node sub-module with NodeWarmer plugin~~ ✓
• ~~Create microwave_taxonomy sub-module with TermWarmer plugin~~ ✓
• ~~Create microwave_commerce_product sub-module with CommerceProductWarmer plugin~~ ✓
• ~~Refactor MicrowaveSettingsForm to build dynamically from discovered plugins~~ ✓
• ~~Define per-sub-module config schema and default config files~~ ✓
• ~~Handle custom URL warming outside of the entity plugin system~~ ✓
• ~~Update Drush commands~~ ✓
• ~~Update README~~ ✓
• Write a migration path / update hook for existing microwave.settings configuration

User interface changes

The settings form is now restructured: instead of a static form partially
built via form alters, it renders one section per discovered warmer plugin.
The visible configuration options remain the same; only the form structure
changed.

API changes

New plugin system introduced: EntityWarmerInterface,
AbstractEntityWarmer, EntityWarmerManager, and the
#[WarmableEntity] attribute. Third-party modules can register a
new warmable entity type by placing a class in their
Plugin/EntityWarmer/ subdirectory — no hooks, services, or form
alters needed.

Drush: the per-entity-type commands have been removed and replaced by the
unified drush microwave:process_entity_urls (drush mpeu).

Data model changes

Configuration is split from the single microwave.settings object
into separate per-sub-module config objects (microwave_node.settings,
microwave_taxonomy.settings,
microwave_commerce_product.settings). An update hook is still
needed to migrate existing site configuration.