Add control of PMD execution threads to plugin goal

[harrisric]

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This change adds feature #380
Tested locally but I have not yet written any unit tests as I was not sure of the style/approach that would be considered suitable for this type of additional configuration. as it doesn't change the output of the analysis/report in anyway. It would be easy to separate the handling of the configuration and add tests for that if required.

[harrisric]

@adangel what needs to happen for this to be reviewed and accepted?

[slawekjaranowski]

A test will be appreciated for it

[harrisric]

@slawekjaranowski happy to do that, could you suggest where this would fit in the existing tests? As I noted above this doesn't change the output of the analysis / report in any way.
I could write a unit test which shows that the numThreadsConverter method works, particularly by separating it from the mojo (and possibly needing to further abstract away the use of Runtime.getRuntime().availableProcessors()).
Or I could write a test which shows that the value is propagated to the PMDConfiguration on the executor, but there are no existing tests like this.
Or I could write a further integration-test, though it would likely be a simple copy of an existing test with the new configuration. Also the variations using a multiple of the cores available are not really testable in this sense as we cannot control the cores available in the test environment.
Just want to make sure any tests I do add here provide some material value to the project.