[loki-canary] Support passing loki user, password and tenant_id as environment variable.

[Vinaum8]

Is your feature request related to a problem? Please describe.
Issue with loki canary authentication.

Describe the solution you'd like
Add support for the following environment variables in the grafana/loki-canary container:

• LOKI_USERNAME: The username for authenticating with the Loki service.
• LOKI_PASSWORD: The password for authenticating with the Loki service.
• LOKI_TENANT_ID: The tenant ID for multi-tenant Loki instances.

Additional context
│ level=error msg="failed to send entry, server rejected push with a non-retryable status code" entry=1719190934246426338 status=401 err="server returned HTTP status 401 Unauthorized │