Add skeleton spec for OP::Plugins::ActsAsOpEngine

Signed-off-by: Alex Coles <alex@alexbcoles.com>
[wip] Add .extend_api_response to ActsAsOpEngine

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Show plugin attributes in WP overview
Merge remote-tracking branch 'origin/feature/api-plugin-extension' into feature/14007_show_backlogs_attributes_in_overview_tab
Fix API representer extension
Fix tests
Use project object extracted by before filter
Don't use WP because it does not exist for index
Fix specs
Merge pull request #1683 from opf/feature/14007_show_backlogs_attributes_in_overview_tab

[Feature] 14007 show backlogs attributes in overview tab