feature: Ability to customize WebSocketGraphQLClient connection init message payload

[gadams00]

Graphql-java-kickstart supports creating a graphql server with a custom authentication interceptor that can inspect the connection message payload for authentication tokens. The DGS WebSocketGraphQLClient has the connection message defined in a companion object with null payload and no possibility to customize, making it incompatible with graphql-java-kickstart servers that use this feature. It would be helpful if it were possible to optionally customize the connect message payload.