Support accepting interceptors in WebSocketGraphQlTester

[mareksvatos]

I am struggling to find a way to set connection_init payload using WebSocketGraphQlTester. It would be nice for WebSocketGraphQlTester to support accepting interceptors in the same way that WebSocketGraphQlClient does.

[rstoyanchev]

It makes sense to expose interceptors on GraphQlTester.Builder which would in turn configure them on the underlying GraphQlClient.

[bclozel]

I had a look and it seems that configuring interceptors on the client is only possible for the WebSocketGraphQlTester, as other variants are dealing with the transport directly. We should probably apply this at the GraphQlTester.Builder level: this means we need to apply this consistently to all tester implementations.

[bclozel]

After discussing this again, we thought that the main use case for interceptors with testers is the one described above. We've added this possibility on the WebSocketGraphQlTester builder only, for now. If we get other requests from the community with concrete use cases, we can reconsider and promote this to a higher level builder.