Add VTAdmin setting for fetch credentials to CRD

[leejones]

We deploy VTAdmin using the vitess-operator and make it accessible using an authenticated proxy. In order for it to work, we need the frontend to send the user's HTTP Cookie containing their credentials when the frontend Javascript makes requests to the VTAdmin API. By default, this is currently disabled by the operator. We'd like the option to change this setting in the CRD (e.g. change to same-origin).

For example, something like this would be great:

kind: VitessCluster:
spec:
  vtadmin:
    fetchCredentials: same-origin

The default could remain omit for backwards compatibility and/or security.

Currently we workaround this missing setting by mounting a custom nginx config that serves a custom config.js instead of the one rendered by the operator.

References:

• Docs on related Javascript Fetch API