Add varargs overload to Assert#hasString and Assert#doesNotHaveString

[ascopes]

This overload will delegate to String#format internally, similar to how StringAssert#isEqualTo allows for string templating expansion. This removes the need to write manual formatting in tests that need to consume a fixture placeholder.

Example:

@Test
void toStringReturnsTheExpectedValue() {
  // Given
  var fileRepository = someFileRepository();
  var fileManager = new FileManager(fileRepository);

  // Then
  assertThat(fileManager)
      .hasToString("FileManager{repository=%s}", repository);
}

...rather than the current solution:

@Test
void toStringReturnsTheExpectedValue() {
  // Given
  var fileRepository = someFileRepository();
  var fileManager = new FileManager(fileRepository);

  // Then
  assertThat(fileManager)
      .extracting(FileManager::toString, STRING)
      .isEqualTo("FileManager{repository=%s}", repository);
  // or
  assertThat(fileManager.toString())
      .isEqualTo("FileManager{repository=%s}", repository);
}