Allow capturing JFrog CLI output

[yahavi]

• This pull request is created in the jfrog/jenkins-jfrog-plugin repository.

Fix jenkinsci#11

Allow capturing command outputs by returning a string result instead of void. This change ensures that stdout is printed to the log regardless of whether the output is captured.

Usage Example:

String version = jf '-v'
echo "JFrog CLI version output: $version"

In a Declarative Pipeline:

Wrap the previous example in a script block:

script {
    String version = jf '-v'
    echo "JFrog CLI version output: $version"
}