[feature]: Allow inline creation of dropdown options in custom properties

Is there an existing issue for this?

• I have searched the existing issues

Summary

When editing a work item, users should be able to type a new value in a custom property dropdown and create that value directly from the work item form, similar to how labels can be created and applied inline.

Example:

• Custom property: Customer
• Existing options: Customer A, Customer B
• User types: Customer C
• Plane shows: Create "Customer C"
• On confirmation, Plane creates the dropdown option and applies it to the work item.

Why should this be worked on?

Custom properties are useful for structured data, but dropdown options currently need to be managed separately. This makes fast classification harder when teams need controlled fields with flexible option creation.

Labels already support fast inline creation, but labels are not a replacement for structured fields. Teams need the same speed of entry while keeping data separated by property, such as Customer, Module, System, Environment, Product, Department, or Validation Status.

This would reduce friction during issue creation, migration, triage, and support workflows, while preserving the structured nature of custom properties.