feat: add support for fetching subgraph schema via http

[raffis]

Current situation

Only inline schema is supported.

Proposal

Adds support for http fetch:

apiVersion: apollo.infra.doodle.com/v1beta1
kind: SubGraph
metadata:
  name: users
spec:
  endpoint: http://user-server/graphql
  schema:
    http:
      endpoint: http://user-server/graphql-schema