Ability to configure Response Cache's `private_id` without a Rhai script

[radekmie]

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

Setting up custom Rhai scripts is not trivial in almost all setups, since it involves an additional volume, etc. Most configuration is done via the YAML file, but the private_id requires a Rhai script to set something on the request's context.

Describe the solution you'd like

For us, the following would be ideal:

response_cache:
  enabled: true
  subgraph:
    all:
      enabled: true
      redis:
        urls: ["redis://..."]
    subgraphs:
      accounts:
        # Based on a header
        private_id:
          header: authorization
        # Based on a JWT field 
        private_id:
          jwt: claim

Describe alternatives you've considered

We could just spend some time to configure the Rhai scripts.

Additional context

None.