Cannot build a conditional expression with an existing schema field in the schema editor

## Bug Summary

When building a field condition, its not possible to reference an existing field in my schema and can only be created with new fields.

## Steps to Reporduce

1. Create a Enum field,
1. key: isHungry,
2. description: Are you hungry?
3. values: Yes, No
2. Add a String type
1. key: meal
2. description: What will you order?
3. Add a condition (to either the enum or string) try to create a condition that shows the `meal` field when `isHungry` is 'Yes\`. There is no option to reference existing fields, only to create a new field or choose a sub-schema

## Expected Result

_Abiltity to choose an existing field (or subschema) from the dropdown._

## Actual Result

_Can only create new fields and select defined sub-schemas_

## Artifacts

https://www.loom.com/share/2dac7a79bf214beba9fd695d9fe54e2b

## Related

#6805