Support configuring application_name in database.yml

By default, the Ruby PostgreSQL driver will specify a fallback_application_name based on the process name and use that as part of the appname connection string. This appname gets sent to PgBouncer, which calls SET application_name every time a connection is used by Rails.

To reduce the load on the database, we want to have the ability to disable the setting of application_name. This can be done by setting this value to a blank string. Note that setting the environment variable PGAPPNAME to a blank string does not do the same thing because an empty environment variable is not defined.

We did this in Omnibus already: gitlab-org/omnibus-gitlab!4808 (merged)

Relates to gitlab-org/gitlab#292022 (closed), https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/12054