Support Relay GraphQL standard

# Summary
Add support for the Relay GraphQL standard

# Motivation

Relay provides quite a lot of useful features to front-developers and given that fact that it is almost a de-facto standard, it means many libraries can provide implementation so that its usage can be ever easier. This is expected to drive adoption of tools like DAB.

# Functional Specifications

Relay is based on two core GraphQL "best practitices" that needs to be implemented in order to support Relay:

- [Complete Connection Model](https://graphql.org/learn/pagination/#complete-connection-model)
- [Global Object Identification](https://graphql.org/learn/global-object-identification/)

more details on the Relay site:

- [GraphQL Cursor Connections Specification](https://relay.dev/graphql/connections.htm)
- [GraphQL Server Specification](https://relay.dev/docs/guides/graphql-server-specification/)

It should be possible to decide if to generate a Relay-compatible GraphQL schema or use the default one provided by Data API Builder, but setting a dedicated configuration option (in the `runtime` section) in the configuration file.