[REQ] [Java-okhttpgson]: Make API response headers case insensitve

[saigiridhar21]

Is your feature request related to a problem? Please describe.

As HTTP headers are case insensitive, it is better to make API response's header map as case insensitive. This is a good to have feature.

Describe the solution you'd like

TreeMap supports usage of comparer. So, we can use TreeMap for API response's headers

Describe alternatives you've considered

Additional context