Add support for file:// protocol in plugin URI

[gcuisinier]

Allow to install plugin from local folder with the file:// format, linked to #365

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Refactored the PluginManager.downloadToFile function to add condition on the URL format to manage HTTPs/HTTP and file protocol.
Extract the current http fetch into a dedicated internal function.

[timja]

Looks fine just needs checkstyle issues sorted

[gcuisinier]

👍
I've to check where I can find the detail of checkstyle issue to fix them

[timja]

https://ci.jenkins.io/job/Tools/job/plugin-installation-manager-tool/job/PR-399/2/checkstyle/

[timja]

This failed when releasing with two spotbugs errors, not sure why it didn't show up initially, are you able to take a look?
https://github.com/jenkinsci/plugin-installation-manager-tool/runs/4665728379

[gcuisinier]

Sure! I will check

[gcuisinier]

Both issue are complaint about the fact that we are trying to use a URL ( http or file ) that's created from an user parameter... and so unsafe.

But for the first (http), it was already present, but in another function (as I just extracted the code to a new function).
For the second, it's about the usage of File from the input param also.

For me there are both false positive, as that's the expected behaviour no?

[timja]

yes you can create a PR to suppress it then

[gcuisinier]

Done here : #400