Add two new methods to Request:

• WithReporter(Reporter)
• WithAssertionHandler(AssertionHandler)

WithReporter(reporter) should be equivalent to:

WithAssertionHandler(DefaultAssertionHandler{
    Reporter: reporter,
    Formatter: r.config.Formatter,
})

WithAssertionHandler should update r.chain.handler with given handler. For that, we'll need to add new method chain.setHandler.

New methods should have documentation and tests. In tests, we should check that after these calls, request and it's newly created children (i.e. Response, its body, etc) start using new handler to report failures.

For that, we can create a mock reporter, pass it to request using WithReporter or WithAssertionHandler, then perform an assertion that will fail (req.Expect()....) and then check that the mock reporter is in failed state.