allow to configure the withCredentials field

[jes2ica]

Our service needs to use credentials: “include" to in fecth() but currently withCredentials is hardcoded to false:

promlens/app/src/pages/PromLens.tsx

Line 157 in 171530e

withCredentials: false,

Can we add a flag similar to defaultPrometheusURL to allow users to configure the withCredentials field?