Added pytest to check correctness of the link authentication pipeline.
Context: this PR is the first step towards refactoring the link authentication pipeline to use https (instead of psql) to send the db info to the proxy. There was a test missing for this pipeline in this repo, so this PR adds that test as preparation for the actual change of psql -> https.
Co-authored-by: Bojan Serafimov <bojan.serafimov7@gmail.com>
Co-authored-by: Dmitry Rodionov <dmitry@neon.tech>
Co-authored-by: Stas Kelvic <stas@neon.tech>
Co-authored-by: Dimitrii Ivanov <dima@neon.tech>