Add a Backend for PeeringDB

[jof]

Expected behaviour

It should be possible to authenticate with PeeringDB.

PeeringDB is a community project used by Internet network operators, Internet Exchanges, and colocation facilities to describe interconnection policies, locations, and contact information.

As a developer of an Internet Exchange user portal, I would like to enable users to log in with PeeringDB.

References

• https://docs.peeringdb.com/oauth/
• https://docs.peeringdb.com/presentation/euroix-201910-euroix35-oauth-for-ixps.pdf