Adds settings to control server CORS configuration

[desertaxle]

Adds the following settings to allow modification to the CORS configuration of a Prefect server:

• PREFECT_SERVER_CORS_ALLOWED_ORIGINS
• PREFECT_SERVER_CORS_ALLOWED_METHODS
• PREFECT_SERVER_CORS_ALLOWED_HEADERS

Closes #15074

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

[codspeed-hq]

CodSpeed Performance Report

Merging #15396 will not alter performance

_{Comparing cors-settings (a69266e) with main (b8bb495)}

Summary

✅ 3 untouched benchmarks