Support equivalent of failurePriority

[adangel]

You can configure a specific priority for each rule in your own ruleset (https://pmd.github.io/latest/pmd_userdocs_configuring_rules.html#message-and-priority-overriding).

The maven-pmd-plugin supports the parameter failurePriority which can used to fail the build only for "high priority" violations. The lower priority violations are still reported, though.

Currently the action only provides the number of found violations regardless of the priority. So you can only fail the build if any violation is found. This probably has to be tackled together with #30 .