vdk-core: credentials for named databases and ingesters

[DeltaMichael]

Prerequisites

#3223
#3224

Overview

Pre-loading database or ingesters requires sensitive configs, like credentials. These should be added in the initialize job hook and fetched either from the secrets api or config.ini.

@hookimpl
  def vdk_initialize(self, context: CoreContext) -> None:
      context.register_db(key="default", type="oracle", host="localhost", port="1521", sid="free")
      context.register_db(key="oracle_1", type="oracle", host="http://some_host_whatever/", port="1522", sid="not_free")
      context.register_db(key="impala_1", type="impala", host="http://impala_host/", database="mydatabase")

@hookimpl
  def initialize_job(self, context: JobContext) -> None:
      context.add_credentials("oracle1", context.core_context.config.get_value("oracle_1", "username"), context.core_context.config.get_value("oracle_1", "password"))

We should fetch credentials from secrets, but we should also be able to configure them in config.ini

Acceptance criteria

1. Propose a way to configure credentials for pre-loaded databases/ingesters
2. Implement proposal, or open a separate implementation ticket