Remove the `customError` function and `CustomValidationError` type.
These were made obsolete by support for returning plain object literals
as custom errors.
This also catches few `field` properties that were missed in the
renaming to `fieldTree`.
This PR makes a number of changes to the metadata API to address design
flaws in the previous API. Some of the changes include:
- Replaces the previous `MetadataKey` and `AggregateMetadataKey` with a
single unified `MetadataKey` that is used for all metadata.
- The new `MetadataKey` is only defined for fields that explicitly set
it in their schema logic
- All metadata now has reducer / aggregate behavior
- The new `MetadataKey` has an option to create a managed key which
wraps the result of its computed aggregate into some other structure
such as a `Resource` or `linkedSignal`
- There are now two APIs to create metadata keys
- `createMetadataKey` for pure computed metadata
- `createManagedMetadataKey` for metadata that manages its computation
internally
