Fix non-editable HTTP headers Grid

[christianhelle]

This pull request includes changes to the AddCustomHeaderDialog class in the ApiClientCodeGen.VSIX.Shared project. The main changes involve modifying the CustomHeader type to use a class instead of a record and updating how CustomHeader instances are created and used.

Changes to AddCustomHeaderDialog class:

• Modified the CustomHeader type from a record to a class, allowing the properties Key and Value to be settable.
• Updated the instantiation of CustomHeader objects to use object initializer syntax for setting Key and Value properties

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud