Closes gh-23758

The infix form limits the extensibility of the API and
prevents calling `andReturn()`.

See gh-1951

This commit introduces a `MockMvc` Kotlin DSL via a set of
extensions like `MockMvc.get` or `MockMvc.request`
that provide a Kotlin idiomatic and easily discoverable
API while staying close to the original Java API design.

Closes gh-1951