Improve database client options templating

[joachimBurket]

The sslrootcert parameter specifies the name of a file containing SSL certificate authority (CA) certificate(s). If the file exists, the server's certificate will be verified to be signed by one of these authorities (in the case of an SSL connection to the PostgreSQL DB).

See the option in the documentation here: https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNECT-SSLROOTCERT

Resolves #417

[LeoColomb]

Thanks for submitting this pull request, @joachimBurket.
I'd like to suggest another approach.
I think it would be more future-proof and more flexible for other engines to render the values directly under a new .Values.externalDatabase.options.

It would be something like that:

      OPTIONS: {{- include "common.tplvalues.render" (dict "value" .Values.externalDatabase.options "context" $) | nindent 8 }}

What do you think?
That would help to cover the bunch of potential parameters: https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PARAMKEYWORDS
/cc @RangerRick

[RangerRick]

@LeoColomb yeah, I like this; should be pretty easy to implement and avoids a TON of boilerplate on the user's part

[RangerRick]

and allows you to decide how to set sslmode, whether you want to set up client certificates as well, etc.

[joachimBurket]

Sorry for the delay, I haven't seen the notification. Yes this seems a way better idea! 👍

[RangerRick]

LGTM, I've also added this to our 5.0.0 release notes WIP:

* The `externalDatabase.sslMode` setting has been removed and replaced by an `options` map that can take an arbitrary list of extra PostgreSQL options.