Make username also an env var for poller

[Sepehr-A]

Added:

• Dynamic username prompt support in credential loader when the username is set to 'ask'.
• MockGetPass class in tests to simulate user input for credentials.
• the documentation for the new environment variable setting (username)

Changed:

• Updated validate_sens_field in StaticModel to include the username.
• Enhanced tests to cover dynamic input with the new mock class.