Create a spring-graphql-client library

[JacobMountain]

It would be good to move some of the code out of the example-client. Possible candidates for this are:

• RestTemplateFetcher.java
• WebClientFetcher.java
• WebSocketSubscriber.java (needs some work as I believe it currently only supports one subscription at a time

[JacobMountain]

Also need to figure out how to properly handle the TypeReference/ParameterizedTypeReferences that spring uses internally to deserialize to the generated response classes.