Implement LDAP Authentication

[AndrewJackson2020]

This PR implements trust (no password), and LDAP authentication with pgcat. Postgres offers a number of authentication options that are not available with pgcat with trust being the most basic. Added a configuration item to allow the auth_type to be set at the user level.

[AndrewJackson2020]

@drdrsh any chance that this functionality could get merged in? Happy to make any changes, add documentation, tests, split up the PR(one for trust, one for LDAP), etc.

[drdrsh]

I haven't had the time to review this yet.
I will take a look later this week or on the weekend.

[drdrsh]

I will appreciate it if you could add some tests.
I'll update the CONTRIBUTING file to show how you can add tests in Ruby/Python/Go/Rust and how you can run them locally.

[drdrsh]

I updated the Contribution guide to show how you can write integration tests.

[AndrewJackson2020]

Thank for updating those docs. Added some basic unit tests for LDAP and trust. Added onto the python unit tests since that is the testing framework that I am most familiar with.

One thing I notice is that the python tests are currently organized as a script, might be worth refactoring it to use pytest to get better reporting, test isolation, parallelism, amongst other things. I opened #790 which modifies the tests to use the pytest framework. If you want to go that route I can change the tests in this PR as well if/when you want to merge that.

Since LDAP requires a dedicated web service I incorporated GLauth in the docker container since it can be installed via a very straightforward binary. GLauth also provides a docker container as well (https://hub.docker.com/r/glauth/glauth) if this fits better with the pgcat testing framework.

[AndrewJackson2020]

I see the #790 was merged. Will work on converting these tests to use pytest today.

[AndrewJackson2020]

Just a heads up, removed the trust auth (passwordless) stuff out of this PR. Will raise a new PR with that stuff. I figure its easier that way as the LDAP functionality requires changes to the dockerfile (and thus the CI) in order to run whereas the trust functionality is a lot more limited in scope.