Add skip configuration for maven plugin

[Abderrahime FARHANE]

actually there is 4 configuration parameters failOnChanged failOnIncompatible newSpec and oldSpec.
we need new parameter to skip the maven goal.

[Abderrahime FARHANE]

 @Parameter(property = "skip", defaultValue = "false")
  Boolean skip = false;

  @Override
  public void execute() throws MojoExecutionException, MojoFailureException {
    if(Boolean.TRUE.equals(skip)) {
      getLog().info("Skipping openapi-diff execution as per configuration");
      return;
    }
    ...

[Jochen Schalanda]

@FARHANE Thanks for reporting this!

It looks like you already have half the pull request ready. 😉