Make ETS working with secured instances

[gfenoy]

Is your feature request related to a problem? Please describe.

We would like to be able to set specific headers to be sent to the Instance Under Test (IUT) for ensuring that server implementation requiring secured access can be tested.

Describe the solution you'd like

Using a new extra_headers option giving the opportunity to pass any extra headers required to IUT.

[ghobona]

Related to opengeospatial/teamengine#46

[fmigneault]

I would love to have this as well!

I was able to set up special permissions set for the ETS to work on endpoints under /processes/echo.
However, the /jobs endpoint can contain other users' jobs, and I cannot open this publicly only for the purpose of the ETS.

If it could support some kind of Authorization: Bearer <token> header, which could be specified as input just like the echo process name can, then I could grant it special access for specific tests and jobs.

[ghobona]

This issue is potentially related to opengeospatial/teamengine#580