[DATA GRID] Avoid schema detection on columns which already have a schema

[chandlerprall]

Summary

Fixes #2460

Avoids running schema-detection on columns which already have a schema.

Checklist

- [ ] Checked in dark mode
- [ ] Checked in mobile
- [ ] Checked in IE11 and Firefox
- [ ] Props have proper autodocs
- [ ] Added documentation examples
- [ ] Added or updated jest tests

• Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[thompsongl]

LGTM. Using Set was neat

[snide]

Hmm. I'm still seeing this broken? To replicate

Go to the schema docs

• Note that the datetime column still shows as a string.
• Also note that the icon no longer shows for the custom schema. It should be a star.

[chandlerprall]

@snide thanks! Fixes for those are pushed - only the detected schema was passed to the sorting dropdown, instead of the whole detected+provided schema.

[snide]

Works now!