Ability to skip generating an index.ts file in the client-preset

[Xmaxer]

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

The index barrelling of the generated files is slightly too opinionated for my liking. Some projects like to have barrelling, some don't. In our projects we have rules to specifically avoid barrelling, so would like to avoid creating exceptions.

Describe the solution you'd like

Create a configurable option to remove index file generation in the client preset.

Describe alternatives you've considered

No response

Any additional important details?

I already have the changes done and tested locally in my own projects: master...Xmaxer:graphql-code-generator:master

If the general idea is ok, I can create a PR.