Enable running CI with auto_explain enabled.

[emelsimsek]

In order to make sure Citus works fine with auto_explain, let's enable running CI with auto_explain enabled. Here is an example configuration:

postgresql.conf
shared_preload_libraries = 'citus,pg_stat_statements,auto_explain'
auto_explain.log_min_duration = 0
auto_explain.log_analyze = true
auto_explain.log_buffers = true
auto_explain.log_nested_statements = true

auto_explain comes built-in with postgresql. No need for additional install.

As an important note, those new tests in CI, as they are likely to fail, should not block github merge workflow for PRs at the moment.

[aykut-bozkurt]

After discussion with @emelsimsek, we decided to support that at citus repo for now.
But we also have a plan to run citus tests with any other extensions created with the help of extension tests automation.
You can follow the issue #262 to be aware of the task status.