Support author name in LicenseHeaderStep

Some license texts include the name of the file's initial creator. This is currently not possible to do with the `LicenseHeaderStep`.
Maybe the step could should work with templates rather than constant strings, e,g,
```groovy
licenseHeader('/* Copyright ${year} ${author}*/',
[year: java.time.LocalDate.now().year, author: 'John Doe'])
```

Start at the LicenseHeaderStep entry point and trace how licenseHeader currently accepts constant strings. Define how the proposed year and author values should be supplied and determine the expected rendered header using the example; completion requires focused coverage for template substitution.

Written by the indexing model from the issue text.