Support customization of the Keystone Admin UI Apollo Client

[acburdine]

Working with a number of different keystone applications in the same project, I discovered another admin customization use-case that might be useful to support in Keystone natively.

We have a use-case in our keystone applications where the Admin UI Apollo Client needs to add additional headers to requests made to the graphql endpoint. At present, there's no way to customize the apollo client link used in the Admin UI, so I ended up making a patch locally that adds a new customApolloLink field to the AdminConfig type, like so:

export type AdminConfig = {
  components?: {
    Logo?: (props: {}) => ReactElement
    Navigation?: (props: NavigationProps) => ReactElement
  },

  customApolloLink?: (link: ApolloLink) => ApolloLink
}

This addition (and the requisite change to the KeystoneProvider code in admin-ui/context) allowed us to customize the link passed to the apollo client. It was a relatively simple change, so I was curious if there'd be interest in supporting something like this in Keystone itself.