Provide mock implementation of `org.springframework.web.servlet.function.ServerRequest`

[vpavic]

Testing org.springframework.web.servlet.function.HandlerFunction implementations would be easier if spring-test would provide a mock implementation of org.springframework.web.servlet.function.ServerRequest, analogous to the org.springframework.mock.web.reactive.function.server.MockServerRequest (that implements the reactive counterpart org.springframework.web.reactive.function.server.ServerRequest).

[PeguB]

Hello, I would like to claim this issue.
I made a PR in advance that includes the required changes.

[bclozel]

Superseded by #31257