Allow playground settings customization

[maimas]

Allow playground settings to be customized from the application.properties

I run in to an issues with this property request.credentials where I was getting 403 errors.
Than I realized that GUI has it's own config view so I had to update the property manually....

Default GUI conf: 'request.credentials': 'omit'
My updated GUI conf: 'request.credentials': 'include'

The problem is that once I deploy my service, other developers will use it's GUI and they will not be aware that there is a setting that needs to be changed from UI. They will face 403 errors and scratch their heads why.

The proposal is to have GUI settings configurable in the applciation.properties so that the service owner can specify the behavior of the UI. I can see this ability in other GQL projects

In my case I always want to have property set to include ---> 'request.credentials': 'include'