[graphiql] optional fetcher, user network configuration tab

A long requested feature, and what we envisioned for the sidebar plugin ecosystem a few years ago, is a place where users can configure their environments and settings for each environment

the network tab would provide a global setting for:

• default initial headers for each operation (that would show up in the headers tab)
• HTTP POST or GET for queries? http spec
• whether to persist the headers for that environment
• whether to add Accept: application/graphql-response+json transparently

the network tab would provide per-environment settings for:

• url
• default initial headers for each operation (that would show up in the headers tab)
• HTTP POST or GET for queries? http spec
• whether to persist the headers for that environment

How will this impact GraphiQL?

• fetcher becomes optional if environments prop is provided
• environments prop looks like graphql-config object but maybe stricter (difficult to normalise every pattern)
• @graphiql/toolkit createGraphiQLFetcher() becomes an internal detail if environments prop is provided?