Add UsingStore<T> declarative enrichment from ancillary stores (#4300)

[erdtsieck]

Summary

Implements the Marten side of declarative ancillary store enrichment (closes #4300).

• Adds AncillaryStoreEnrichmentExtensions.UsingStore<TStore>() — an extension on SliceGroup.EntityStep<TEntity> that redirects enrichment lookups to a Lazy<TStore> ancillary store
• Opens a lightweight session per enrichment call; session lifecycle (dispose) is handled automatically via WithAlternateSession from JasperFx.Events
• Caching is preserved: the built-in upstream aggregate cache is checked before hitting the ancillary store
• Tenant-aware: FetchProjectionStorageAsync handles tenant routing internally

Usage

public override async Task EnrichEventsAsync(
    SliceGroup<Order, Guid> group,
    IQuerySession querySession,
    CancellationToken cancellation)
{
    await group.EnrichWith<Product>()
        .UsingStore(_productStore)          // Lazy<IProductStore>
        .ForEvent<OrderPlaced>()
        .ForEntityId(e => e.ProductId)
        .EnrichAsync((slice, e, product) => e.Data.Product = product);
}

Dependencies

Requires JasperFx.Events WithAlternateSession support: JasperFx/jasperfx#195

Closes #4300

[jeremydmiller]

Just moved this into a different PR