* Customizing HTTP headers in the config file
* Add changelog, fix bad imports
* fixing some bugs
* fixing interaction of custom headers and /ui
* Defining a member in core to set custom response headers
* missing additional file
* Some refactoring
* Adding automated tests for the feature
* Changing some error messages based on some recommendations
* Incorporating custom response headers struct into the request context
* removing some unused references
* fixing a test
* changing some error messages, removing a default header value from /ui
* fixing a test
* wrapping ResponseWriter to set the custom headers
* adding a new test
* some cleanup
* removing some extra lines
* Addressing comments
* fixing some agent tests
* skipping custom headers from agent listener config,
removing two of the default headers as they cause issues with Vault in UI mode
Adding X-Content-Type-Options to the ui default headers
Let Content-Type be set as before
* Removing default custom headers, and renaming some function varibles
* some refacotring
* Refactoring and addressing comments
* removing a function and fixing comments
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