Support for copying file from Jenkins host to agents

[jonkipu]

This change adds a config option for the host to allow selecting Jenkins File Credentials items to be uploaded to a path relative to the agent's User home folder, this was useful to me in some cases where I was using tools requiring specific config files including password / tokens etc (.npmrc, .aws config, ssh config)

Pretty similar to #15 but I needed it to be more generic

Unsure to write tests for such a feature since I'm pretty new to Jenkins plugins, would love some guidance in this area

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[mat1e]

Hi @jonkipu, the code looks clean thank you. Please add somes tests, especially for the new method uploadHostFile. I will try your feature and let you know.

[jonkipu]

Thanks I will try to add some tests, also I want to fix a possible security issue with the path resolving to make sure you can't "escape" the home folder. Will update you when I'm done