Node authors may now create their own arbitrary/custom field types. Any pydantic model is supported.

Two notes:
1. Your field type's class name must be unique.

Suggest prefixing fields with something related to the node pack as a kind of namespace.

2. Custom field types function as connection-only fields.

For example, if your custom field has string attributes, you will not get a text input for that attribute when you give a node a field with your custom type.

This is the same behaviour as other complex fields that don't have custom UIs in the workflow editor - like, say, a string collection.

We need to hold onto the original type of the field so they don't all just show up as "Unknown".

In the initial commit, a custom field's original type was added to the *field templates* only as `originalType`. Custom fields' `type` property was `"Custom"`*. This allowed for type safety throughout the UI logic.

*Actually, it was `"Unknown"`, but I changed it to custom for clarity.

Connection validation logic, however, uses the *field instance* of the node/field. Like the templates, *field instances* with custom types have their `type` set to `"Custom"`, but they didn't have an `originalType` property. As a result, all custom fields could be connected to all other custom fields.

To resolve this, we need to add `originalType` to the *field instances*, then switch the validation logic to use this instead of `type`.

This ended up needing a bit of fanagling:

- If we make `originalType` a required property on field instances, existing workflows will break during connection validation, because they won't have this property. We'd need a new layer of logic to migrate the workflows, adding the new `originalType` property.

While this layer is probably needed anyways, typing `originalType` as optional is much simpler. Workflow migration logic can come layer.

(Technically, we could remove all references to field types from the workflow files, and let the templates hold all this information. This feels like a significant change and I'm reluctant to do it now.)

- Because `originalType` is optional, anywhere we care about the type of a field, we need to use it over `type`. So there are a number of `field.originalType ?? field.type` expressions. This is a bit of a gotcha, we'll need to remember this in the future.

- We use `Array.prototype.includes()` often in the workflow editor, e.g. `COLLECTION_TYPES.includes(type)`. In these cases, the const array is of type `FieldType[]`, and `type` is is `FieldType`.

Because we now support custom types, the arg `type` is now widened from `FieldType` to `string`.

This causes a TS error. This behaviour is somewhat controversial (see microsoft/TypeScript#14520). These expressions are now rewritten as `COLLECTION_TYPES.some((t) => t === type)` to satisfy TS. It's logically equivalent.

…types

- Update connection validation for custom types
- Use simple string parsing to determine if a field is a collection or polymorphic type.
- No longer need to keep a list of collection and polymorphic types.
- Added runtime checks in `baseinvocation.py` to ensure no fields are named in such a way that it could mess up the new parsing

…onStartFieldType'

This was confusingly named and kept tripping me up. Renamed to be consistent with the `reactflow` `ConnectionStartParams` type.

"Custom", "CustomCollection" and "CustomPolymorphic" are reserved field names.