Fix: Accept custom pgxpool

[vr-varad]

Fixes #87

[vr-varad]

@maxdml Kindly review at your convenience.

[maxdml]

@vr-varad thank you for your contributions! Two main comments:

• The custom pgxpool should be provided by the end-user, as a new argument to the Config of DBOSContext. I didn't describe Accept custom pgxpool #87 well enough, sorry about that. We'll pass the user-provided through to newSystemDatabase, as you are doing now.
• We need to tests that the custom pool works. For this we can add a new test in dbos_test.go (e.g., TestCustomSysDBPool) that will create a new DBOSContext with a custom pgx pool, then do something simple, like running a workflow with a step. You can look at workflows_test.go to see how its done.

[vr-varad]

@maxdml Made the required changes, kindly review.

[maxdml]

Thanks @vr-varad , left a few comments. We're getting there!

[vr-varad]

Hey @maxdml made the changes accordingly!!!

[vr-varad]

Done!! @maxdml
and test passed as well!!!

[vr-varad]

Any Update on this @maxdml

[maxdml]

lgtm, thanks @vr-varad