Add the ability to use Fragments and post-execution rules with the single server/schema

[dimatill]

https://www.the-guild.dev/graphql/shield/docs/advanced/fragments

Some options:

• Simply wrap the schema to create a proxy schema and assign rules to the proxy schema. In such a way all rules assigned to non-root fields will be executed after resolvers of the original schema.
• Hook contextual rules to the original schema; Hook rules that depend on the parent to the proxy schema to have an ability to use Fragments.