Jumoo.Processing: opt-in IProcessingOptions type registry

Jumoo.Processing: implemented step 1 of the polymorphic-discriminator fix designed in #22 (see 2026-09-07 entry) — IProcessingOptionsTypeRegistry, IUmbracoBuilder.AddProcessingOptionsType<T>() and [ProcessingOptionsDiscriminatorAttribute], so a consumer can explicitly register its IProcessingOptions type and sidestep the ITypeFinder scan that misses it when two physical copies of Jumoo.Processing.Core are loaded
Zero breaking changes — an unregistered type keeps using the old scan and class-name discriminator, so nothing already persisted in Jumoo_Pipelines is affected. Merged in #24
Translation Manager and uSync.Complete adopting the new registration API is tracked as follow-up work