Support using access credentials in Prometheus provider

[bacherfl]

Currently the prometheus provider only supports accessing an unprotected prometheus instance. The goal of this ticket is to enable the prometheus provider to accept the reference to a secret containing username/password authentication, similar to how the dynatrace and datadog providers also make use of that mechanism.

DoD

• Prometheus provider can make use of user/password authentication by providing a secret containing those credentials
• Unit tests
• Integration test check with mock
• Document prometheus provider change