Change dataset URI validation to raise warning instead of error in Airflow 2.9

[tatiana]

Closes: #39486

Context

Valid DAGs that worked in Airflow 2.8.x and had tasks with outlets with specific URIs, such as Dataset("postgres://postgres:5432/postgres.dbt.stg_customers"), stopped working in Airflow 2.9.0 & Airflow 2.9.1, after #37005 was merged.

This was a breaking change in an Airflow minor version. We should avoid this.

Airflow < 3.0 should raise a warning, and from Airflow 3.0, we can make errors by default. We can have a feature flag to allow users who want to see this in advance to enable errors in Airflow 2. x, but this should not be the default behaviour.

The DAGs should continue working on Airflow 2.x minor/micro releases without errors (unless the user opts in via configuration).

How to reproduce

By running the following DAG with apache-airflow==2.9.1 and apache-airflow-providers-postgres==5.11.0, as an example:

from datetime import datetime

from airflow import DAG
from airflow.datasets import Dataset
from airflow.operators.empty import EmptyOperator



with DAG(dag_id='empty_operator_example', start_date=datetime(2022, 1, 1), schedule_interval=None) as dag:

    task1 = EmptyOperator(
        task_id='empty_task1',
        dag=dag,
        outlets=[Dataset("postgres://postgres:5432/postgres.dbt.stg_customers")]
    )

    task2 = EmptyOperator(
        task_id='empty_task2',
        dag=dag
    )

    task1 >> task2

Causes to the exception:

Broken DAG: [/usr/local/airflow/dags/example_issue.py]
Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/airflow/datasets/__init__.py", line 81, in _sanitize_uri
    parsed = normalizer(parsed)
             ^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/airflow/providers/postgres/datasets/postgres.py", line 34, in sanitize_uri
    raise ValueError("URI format postgres:// must contain database, schema, and table names")
ValueError: URI format postgres:// must contain database, schema, and table names

About the changes introduced

This PR introduces the following:

1. A boolean configuration within [core], named strict_dataset_uri_validation, which should be False by default.

2. When this configuration is False, Airflow should raise a warning saying:

From Airflow 3, Airflow will be more strict with Dataset URIs, and the URI xx will no longer be valid. Please, follow the expected standard as documented in XX.

3. If this configuration is True, Airflow should raise the exception, as it does now in Airflow 2.9.0 and 2.9.1

4. From Airflow 3.0, we change this configuration to be True by default.

[pankajkoti]

Since we have multiple approvals on this, I'm merging this. However, please drop a comment here in case someone has comments and we can address those in a subsequent PR.