Support of config bundles with extra configuration files

[romani]

https://github.com/checkstyle/test-configs/tree/main/RegexpHeader/Example2
requires extra config file
<property name="headerFile" value="config/java.header"/> (folder is not required we can update example to be just java.header)

test-configs/RegexpHeader/Example2/config.xml

Line 20 in 6700899

<property name="headerFile" value="config/java.header"/>

Even we add it 24f8a55 like this.
it will not be downloaded, as we download only 2 files from test-configs repo.
and inside regression-report.yml of checkstyle we copy them to required folder for report generation.
https://github.com/checkstyle/checkstyle/blob/5eca53ce31f48b61fae0612a2092815009772b48/.github/workflows/regression-report.yml#L388-L394

We need to find a way to download all required files/folders and copy them to execution folder by regression-report.yml .

This feature will help a lot of modules, as several of them have ability to keep configs in separate files.

[romani]

Proporsed solution: we can make extra file in Example folder of this repo: extra-config-files.txt
content of this file will be just list of files in this folder for other to download.
So regression-report.yml workflow will download list-of-projects.yml and config.xml. Download extra-config-files.txt, quickly split all lines in it and download all that referenced in it.
regression-report.yml will cache all downloaded files, even extra-config-files.txt.
regression-report.yml will be smart during copy file from cache to execution folder to be aware of extra-config-files.txt and copy few more files.

ATTENTION:
To simplify first version, lets presume there is no folders of configs, all extra files should be in ExampleX folder.

[romani]

@relentless-pursuit , we missed all config folder

test-configs/Header/all-examples-in-one/config.xml

Line 25 in 31b895f

<property name="headerFile" value="${config.folder}/java.header"/>

Please update extractor to place it in this folder too

[romani]

@relentless-pursuit , thanks a lot for fix, while I am testing it for Header Check, can you make same update for https://checkstyle.org/checks/header/regexpheader.html#RegexpHeader