Add Azure Entra/AD Authentication to PostgreSQL Plugin

**Why is this needed**:
I'm currently working with a customer that mandates PostgreSQL database access via Azure AD Authentication. However, the current PostgreSQL plugin supports only username and password-based authentication.

**What would you like to be added**:
I'm requesting an enhancement that would enable the PostgreSQL Plugin to support Azure AD Authentication for logging in.

**Who is this feature for?**
This feature is primarily intended for Grafana users who need to access PostgreSQL data sources using Azure AD Authentication.

Start by tracing the PostgreSQL plugin's current username-and-password authentication path and identify the entry points involved in establishing a data-source connection. Determine how Azure Entra/AD credentials should be supplied and refreshed, then verify that Grafana users can connect to PostgreSQL without password-based authentication.

Written by the indexing model from the issue text.