Support providing regions/endpoint configuration at connection-time

Currrently, configuration of the secrets manager region and endpoint needs to be done through system properties and environment variables. There are a few problems with this approach that I have run into:

1. If ARNs ever have to be retrieved from different regions, the driver only supports one
2. Setting the environment variables has been an issue in some environments, like when EMR runs your code in yarn containers

If region/endpoint could be provided as connection properties, the region/endpoint could be used dynamically.

Start by locating how the driver currently reads Secrets Manager region and endpoint values from system properties and environment variables, then trace how connection properties are parsed. Done means callers can provide region and endpoint at connection time and those values can vary dynamically for different ARN lookups; verify behavior with the existing test suite.

Written by the indexing model from the issue text.