Bulk write hooks

JSON API doesn't define bulk writes, but that doesn't mean api-pls needs to prevent it. Users should be able to define custom endpoints for their resources for bulk writes.

This shouldn't be too hard:

1. supply the user the tools they need to define any endpoint
2. let them define bulk write handlers in their adapter
3. 🎉 done

Start by tracing how api-pls exposes custom endpoints for resources and how resource adapters currently handle writes; the issue does not name files or tests. Define what adapter hooks and endpoint tools are needed for bulk writes, then verify that a custom resource endpoint can invoke a bulk write handler.

Written by the indexing model from the issue text.