Issue #3609123: Make payable resolvers configurable so the variable resolver names its own variables

Makes PayableResolver plugins configurable (ConfigurableInterface + PluginFormInterface via orchestra's shared ConfigurablePluginTrait), and has the payment step render the chosen resolver's own settings subform, swapping it live when the resolver select changes (reusing InnerPluginSettingsTrait, the same machinery the node interaction editor uses). A resolver with no settings renders nothing.

The variable resolver gains three fields naming the amount, currency and gateway variables it reads, defaulting to payment_amount / payment_currency / payment_gateway. So a workflow can point the resolver at its own variable (e.g. order_total) instead of copying into payment_amount.

Coverage:

• Kernel: the variable resolver honors configured variable names.
• FunctionalJavascript: selecting the variable resolver reveals its fields, switching back to a settings-less resolver removes them, with no orphaned-field validation error.

phpcs / cspell clean. Pre-1.0, no released users, so no upgrade path.

Closes #3609123.