Pluggable ScanItems

[Fryguy]

This PR introduces pluggable scan items in a similar way to dialogs and product features.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1773640

@bdunne Please review

cc @agrare @pemcg @jrafanie

[Fryguy]

Technically this improves seeding performance as well, but barely any :)

	first seed	second seed
before	161.8 ms	111.4 ms
after	104.8 ms	24.0 ms
savings	35.2%	78.5%

[Fryguy]

@pemcg Just wanted to clue you into this one. When you end up adding your scan item to v2v, please submit a PR to remove this skip line.

For now, I couldn't figure out a way to ensure this works without having a plugin bring a scan item. In the future @bdunne suggested we move our default scan items to manageiq-content, drop the core bringing scan items, then this test will just work.

[miq-bot]

Checked commit Fryguy@118170a with ruby 2.4.6, rubocop 0.69.0, haml-lint 0.20.0, and yamllint 1.10.0
3 files checked, 1 offense detected

spec/models/scan_item/seeding_spec.rb

• ❗ - Line 56, Col 68 - Rails/Date - Do not use to_time on Date objects, because they know nothing about the time zone in use.

[simaishi]

Ivanchuk backport details:

$ git log -1
commit 75d79eadf0c5a291e926e6db5557d2d6bccaf5e8
Author: Brandon Dunne <bdunne@redhat.com>
Date:   Mon Oct 14 14:15:39 2019 -0400

    Merge pull request #19388 from Fryguy/pluggable_scan_items
    
    Pluggable ScanItems
    
    (cherry picked from commit c0664f72920d8c50019144288e8dff11466bab23)
    
    Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1773703