[client] update GraphQLWebClient to correctly apply codecs

[dariuszkuc]

📝 Description

Apply default codecs for JSON encoder/decoder without overwriting whole exchange strategies.

🔗 Related Issues

Resolves: #899

[dariuszkuc]

Ideally it would be great if we could do something like create builder with defaults and then apply the builder provided by the user but based on the Spring docs there doesn't appear to be a simple way to do this.

Spring WebClient.Builder currently only expose apply(Consumer<WebClient.Builder> builderConsumer) which could be used to customize the builder but that would also complicate integrating client with Spring apps -> i.e. instead of autowiring beans managed by Spring users would have to explicitly provide lambda to customize the behavior. Unless Spring api changes I think this is probably the "best" we can do atm.