Custom Workflow Instance Attributes

There are scenarios where it would be useful to be able to associate custom data with a workflow instance and be able to query on this as well.

We could do to this by adding a new property to `WorkflowInstance` called `CustomAttributes`, which would be a simple dictionary of string/object pairs.

To make this searchable, we would also introduce a new table called `WorkflowInstanceAttributes`.

This has the potential to solve the following requests:

- https://github.com/elsa-workflows/elsa-core/discussions/1518

Start by reading the WorkflowInstance model and the related request in discussion 1518. Trace how workflow-instance data is persisted and queried before deciding how the CustomAttributes dictionary and WorkflowInstanceAttributes table should fit. Done means custom values can be associated with instances and queried reliably.

Written by the indexing model from the issue text.