Configure JSON GraphQlModule in client support

The various GraphQL clients supported in this project automatically detect
JSON codecs for reading/writing GraphQL requests as JSON payloads.
If there is none detected, clients provide a default codec instance.

This commit configures automatically the `GraphQlModule` from gh-1174 in
default codecs and add integration tests for `FieldValue<T>` usage on the
client side.

This also improves the documentation around `FieldValue<T>` for both
server and client side support.

Closes gh-1190

spring-graphql-docs/modules/ROOT/pages/client.adoc

@@ -393,19 +393,36 @@ include-code::UseInterceptor[tag=register,indent=0]
 
 
 
-[[client.argument-value]
-== Argument Value
+[[client.fieldvalue]]
+== `FieldValue`
 
-If you want to use `ArgumentValue` from a client or test, you can register the
-`GraphQLModule` in Jackson which can serialize/deserialize the value depending on
-the state.
+By default, input types in GraphQL are nullable and optional, an input value (or any of its fields)
+can be set to the `null` literal, or not provided at all. This distinction is useful for
+partial updates with a mutation where the underlying data may also be, either set to
+`null` or not changed at all accordingly.
 
-For example:
+Similar to the xref:controllers.adoc#controllers.schema-mapping.fieldvalue[`FieldValue<T> support in controllers`],
+we can wrap an Input type with `FieldValue<T>` or use it at the level of class attributes on the client side.
+Given a `ProjectInput` class like:
+
+include-code::ProjectInput[indent=0]
+
+We can use our client to send a mutation request:
+
+include-code::FieldValueClient[tag=fieldvalue,indent=0]
+
+For this to work, the client must use Jackson for JSON (de)serialization and must be configured
+with the `org.springframework.graphql.client.json.GraphQlModule`.
+This can be registered manually on the underlying HTTP client like so:
+
+include-code::FieldValueClient[tag=createclient,indent=0]
+
+This `GraphQlModule` can be globally registered in Spring Boot applications by contributing it as a bean:
 
 [source,java,indent=0,subs="verbatim,quotes"]
 ----
     @Configuration
-    public class MyConfiguration {
+    public class GraphQlJsonConfiguration {
 
     	@Bean
     	public GraphQLModule graphQLModule() {

spring-graphql-docs/modules/ROOT/pages/controllers.adoc

@@ -403,7 +403,7 @@ specified in the annotation, or to the parameter name. For access to the full ar
 map, please use xref:controllers.adoc#controllers.schema-mapping.arguments[`@Arguments`] instead.
 
 
-[[controllers.schema-mapping.field-value]]
+[[controllers.schema-mapping.fieldvalue]]
 === `FieldValue`
 
 By default, input arguments in GraphQL are nullable and optional, which means an argument
@@ -426,13 +426,21 @@ For example:
 	@Controller
 	public class BookController {
 
-		@MutationMapping
-		public void addBook(FieldValue<BookInput> bookInput) {
-			if (!bookInput.isOmitted()) {
-				BookInput value = bookInput.value();
-				// ...
+		@QueryMapping
+		public List<Book> searchBook(@Argument String search, FieldValue<Genre> genre) {
+			if (!genre.isOmitted()) {
+				// genre has been set but might hold a "null" value
+				Genre genreValue = genre.value();
 			}
 		}
+
+		@MutationMapping
+		public void addBook(@Argument BookInput bookInput) {
+            FieldValue<String> genre = bookInput.genre();
+            genre.ifPresent(genre -> {
+                //...
+            });
+		}
 	}
 ----
 
@@ -440,6 +448,9 @@ For example:
 method parameter, either initialized via a constructor argument or via a setter, including
 as a field of an object nested at any level below the top level object.
 
+This is also supported on the client side with a dedicated Jackson Module,
+see the xref:client.adoc#client.fieldvalue[`FieldValue` support for clients] section.
+
 
 [[controllers.schema-mapping.arguments]]
 === `@Arguments`