[MPMD-223] Enable specifying file into which report is written

**[Ernst Reissner](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=ernstreissner)** opened **[MPMD-223](https://issues.apache.org/jira/browse/MPMD-223?redirect=false)** and commented

https://pmd.github.io/pmd-5.4.2/usage/ant-task.html
describes the access on pmd via ant.
This is about all which is possible via command line.
I think, the maven plugin should compete with this.

In particular, it is useful to allow to specify the formatter
and toFile.
Currently, not the file is given but the directory to write the file into.
This is not what is needed.

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**Affects:** 3.6

**Issue Links:**
- [MPMD-306](https://issues.apache.org/jira/browse/MPMD-306) PMD and CPD goal must specify output file name when custom renderer specified as format.

Start with the Maven plugin goal and configuration that currently accepts an output directory, then compare its behavior with the referenced Ant task documentation. Trace how the formatter and output path are passed to PMD, and add coverage showing that a specific output file can be selected. Done means the Maven plugin supports both formatter selection and an explicit output file rather than only a directory.

Written by the indexing model from the issue text.