Hotfix - merge cached datamap settings with new data

[TheAndrewJackson]

Closes #3212

Code Changes

• Create new mergeColumns function that handles combining the cached columns with the new columns

Steps to Confirm

• Visit the datamap page so the column settings get cached
• Go create a new custom field
• create a system and populate the custom field with data
• visit the datamap table and open the settings modal. The new custom field should be in the listed columns. NOTE: it won't show up if no system is using the column
• Go back and edit the name of the custom field
• Visit the datmamp table again and check the settings modal. The column should be in the modal with the new name. It will most likely be at the bottom. That is expected.

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation:
  • documentation complete, PR opened in fidesdocs
  • documentation issue created in fidesdocs
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md
• For API changes, the Postman collection has been updated

Description Of Changes

Write some things here about the changes and any potential caveats

[cypress]

Passing run #1760 ↗︎

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Details:

Merge e4d2ab5 into e917207...
Project: fides	Commit: 2bf53d6ccc ℹ️
Status: Passed	Duration: 01:05 💡
Started: May 4, 2023 3:31 PM	Ended: May 4, 2023 3:33 PM

_{This comment has been generated by cypress-bot as a result of this project's GitHub integration settings.}

[adamsachs]

this is looking really good to me in some manual testing locally! things get updated seamlessly as far as i can tell.

i happened to notice a BE regression in doing testing for this, i'll file/follow up with that separately.