[Agent] Handle abs paths on windows correctly

[michalpristas]

What does this PR do?

If absolute path in unix format is used in config, IsAbs on windows returns false. so we first transform it to windows path format and then handle the logic as usual.
For transformation volume of root directory passed is used, it is usually a directory where installed beat is located.

Why is it important?

Supporting windows

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Fixes #17449

[elasticmachine]

Pinging @elastic/ingest-management (Project:fleet)

[ph]

LGTM, lets get the jenkins changes in before so we can test it on CI.

[ph]

I've looked at our lint job and this look like some instability from Jenkins:

04:47:23  java.lang.InterruptedException: no matches found within 10000
04:47:23  	at hudson.FilePath$ValidateAntFileMask.hasMatch(FilePath.java:2826)
04:47:23  	at hudson.FilePath$ValidateAntFileMask.invoke(FilePath.java:2705)
04:47:23  	at hudson.FilePath$ValidateAntFileMask.invoke(FilePath.java:2686)
04:47:23  	at hudson.FilePath$FileCallableWrapper.call(FilePath.java:3073)
04:47:23  Also:   hudson.remoting.Channel$CallSiteStackTrace: Remote call to JNLP4-connect connection from beats-ci-immutable-ubuntu-1604-1585902730876288756.c.elastic-ci-prod.internal/10.224.0.86:56266=

[ph]

backported in #17599