Support for custom text-only attributes in registration app

The config schema for the Registration App allows the creation of field definitions that describes itself as supporting "Definitions for custom fields that can be used in sectionDefinitions".

However, when rendering the actual field, we use this function, which only allows the selection of a fixed number of pre-defined fields. Ideally, the registration app should support configuration-defined fields that can be backed by a simple string and stored as a patient attribute. Basically, this is asking for the same support we have for, e.g., the PhoneField, but for configuration-defined fields.

PR: O3-1054: Support for custom text-based attributes by vasharma05 · Pull Request #132 · openmrs/openmrs-esm-patient-management (github.com)