Add AssertJ support for MockMvc

This commit adds a new way to use MockMvc by returning a MvcResult that
is AssertJ compatible. Compared to the existing MockMvc infrastructure,
this new model has the following advantages:

* Can be created from a MockMvc instance
* Dedicated builder methods for easier setup
* Assertions use familiar AssertJ syntax with discoverable assertions
through a DSL
* Improved exception message

See gh-21178

Co-authored-by: Brian Clozel <brian.clozel@broadcom.com>

spring-test/src/test/resources/org/springframework/test/web/servlet/assertj/message.json

@@ -0,0 +1,3 @@
+{
+  "message": "hello"
+}