Plugin: Support local tgz install & tgz with directory

[steven-sheehy]

What this PR does / why we need it:
Closes #5540

Enhancements:

• Adds support for local installation of tgz files
• Adds support for tgz files with directories (i.e. non-tarbombs)

Bug fixes:

• Ensure gzip reader is closed after use
• Ensure creation of the plugin cache directory is successful before extracting plugin
• Ensure extracted files get created with the correct permission from the tar header (this fixes executable files not having execute permission)

Special notes for your reviewer:

• Moves Extractor to separate file for re-use between local and HTTP installers
• Detects directory of plugin.yaml in extracted hierarchy and adjusts the plugin path accordingly

If applicable:

• this PR contains documentation
• this PR contains unit tests
• this PR has been tested for backwards compatibility

[steven-sheehy]

@thomastaylor312 @bacongobbler Would this be more appropriate for helm v3?

[thomastaylor312]

@steven-sheehy given all of the maintainer's focus on helm 3, I think it probably would be more appropriate for Helm 3

[bacongobbler]

closing as the action item is to implement this for Helm 3.