Frontend changes required by CouchDB 3.1

[amaltaro]

This PR takes over some of the changes initially provided in #1088

NOTE that this is a work in progress!

In short:

• we are defining a new frontend/Apache environment variable that will contain the path to a file with CouchDB user and roles
• the frontend cmsauth module will parse that file and load the user and role in memory
• once an HTTP request is successfully authenticated, two extra HTTP headers will be added before that request is passed over to the backends
  • x-auth-couchdb-username will contain the CouchDB user name
  • x-auth-couchdb-roles will contain the CouchDB user role
• CouchDB will then use this information to authorize the user to access the databases and many other Couch endpoints.

[amaltaro]

This is an almost working feature, however I still see authentication error in the backend. I tried debugging it to see if there was any hidden character together with the credentials, but it doesn't seem to be the case. I also couldn't find a simple way to expand all the headers coming to the backend.

Anyways, I discovered 2 CouchDB parameters that would make its authentication work just like CouchDB 1.6:
admin_only_all_dbs
and
default_security

they have been provided here: #1088 and some initial tests were successful.

[amaltaro]

DO NOT MERGE IT! I am not closing it yet though because we might decide to do something different in the next month or so.

[amaltaro]

@muhammadimranfarooqi Imran, I think this PR can be closed now that we know we don't need these changes. If we ever need it, I can come back to this. Closing it.