[6.x] Fix addon settings blueprint cache collision with field settings blueprint

[duncanmcclean]

This pull request fixes an issue where using addon settings inside a fieldtype's configFieldItems method would cause the addon settings blueprint fields to be returned rather than the field's own settings.

This was happening because Addon::settingsBlueprint() created a Blueprint::make() with no handle. When a fieldtype's configFieldItems accessed addon settings, the applyBlueprintDefaults method (added in #14384) would resolve the addon's settings blueprint, caching its contents and fields under a Blink key based on an empty handle. Later, when the field settings blueprint (also created with Blueprint::make() and no handle) tried to resolve its own fields, it would get the cached addon settings blueprint fields instead.

This PR fixes it by giving addon settings blueprints a unique handle (addons.{slug}), preventing the Blink cache collision.

Fixes #14472
Caused by #14384