Document use of or add subchart for bitnami/postgresql

[diverdane]

Is your feature request related to a problem? Please describe.

The current Helm chart provides "home-grown" versions of YAML templates for
Conjur's Postgresql backend database. It would be much less maintenance for
us to use the open-sourced bitname/postgresql Helm chart that has been
tested and thoroughly vetted by a wide community.

Describe the solution you would like

There are two potential solutions that should be considered:

(1) Remove our current Postgresql YAML support and replace it
with documentation on how to either spin up a Postgresql
backend using the bitnami/postgresql Helm chart,
or use your own external database.
(2) Remove our current Postgresql YAML support and
add the bitnami/postgresql Helm chart as a subchart.

There is a Proof-of-Concept pull request that implements (2):
#83

Describe alternatives you have considered

The alternative is to leave things as is for Postgresql support
in our Helm chart, and continue to maintain this support.

Additional context

Add any other context information about the feature request here.