vdk-core: subsection secrets override

[DeltaMichael]

Prerequisites

#3305
#3306

Overview

If we have a config subsection like

[vdk]
oracle_connection_string=something

[vdk.myoracle]
oracle_connection_string=localhost:1521/FREE

We should be able to override it with secrets

vdk.myoracle_oracle_connection_string=myconnectionstring

We use a dot for the prefix to differentiate with the default [vdk] section and its secrets overrides, e.g.

vdk_oracle_connection_string=something_else

Acceptance Criteria

1. The above scenario is supported
2. Tests added