feature: RestClientGraphQLClient should allow to use common message converters

[GFriedrich]

Describe the Feature Request

Right now the RestClientGraphQLClient either asks for an ObjectMapper or uses the default one from the GraphQLClients object.
The question here is: Why is an additional ObjectMapper needed at all? By default Spring already registers a couple of message converters that take care of converting complex objects to strings for the HTTP request, so I would like to be able to use those converters without any additional ObjectMapper usage.

Describe Preferred Solution

Instead of converting the request at

dgs-framework/graphql-dgs-client/src/main/kotlin/com/netflix/graphql/dgs/client/RestClientGraphQLClient.kt

Lines 85 to 88 in 6450c31

	val serializedRequest =
	mapper.writeValueAsString(
	GraphQLClients.toRequestMap(query = query, operationName = operationName, variables = variables),
	)

the request should be sent "as-is" down to the RestClient if no ObjectMapper is set, so that the common message converters are utilized.
This in turn will cause a slight problem with the GraphQLClientException which then wouldn't be able to get the actual sent request, but I would suggest to simply use the actual request map that was forwarded to the RestClient within the exception. It would also allow for a better way of handling the request details in a potential exception handler.

Describe Alternatives

N/A