v3150: Added external_environment_variable_groups as allowed includes (#3776)

* Added external_environment_variable_groups as allowed includes

For tooling that depends on samson export data there is currently no
good way of pulling what external groups are configured for a project.

Add endpoints to list all external env groups, and allow them as an
include on a projects json export.

* use hooks

* We don't have a html representation of these yet

* enable external environment variable groups for test

* add test for project_allowed_includes

* test both with / without enabled ext var groups

* cleanup extern env vars routes

* clean up test

Co-authored-by: Michael Grosser <michael@grosser.it>