Introduce assorted Reactor `StepVerifier` Refaster rules

[werli]

Summary

This PR introduces a bunch of rules associated to StepVerifier.Asserations and a trivial StepVerifier rule.

---

Example

I spotted the following interesting pattern:

Mono.empty()
    .as(StepVerifier::create)
    .expectError(SpecificException.class)
    .verifyThenAssertThat()
    .hasOperatorErrorWithMessage("msg");

Which made me find the interesting StepVerifier#Assertions API. It's quite low-level, and IMO not as straightforward to read and write.

The example itself is most often represented in our code-base using AssertJ's richer API:

Mono.empty()
    .as(StepVerifier::create)
    .verifyErrorSatisfies(t ->
        assertThat(t)
        .isInstanceOf(SpecificException.class)
        .hasMessage("msg"));

---

Suggested commit message:

Introduce assorted Reactor `StepVerifier` Refaster rules (#1132)

[github-actions[bot]]

Looks good. No mutations were possible for these changes. Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

[github-actions[bot]]

Looks good. No mutations were possible for these changes. Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

[github-actions[bot]]

Looks good. No mutations were possible for these changes. Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

[github-actions[bot]]

Looks good. No mutations were possible for these changes. Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

[github-actions[bot]]

Looks good. No mutations were possible for these changes. Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud