**Apache Airflow version**:


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:
`get_uri()` and `get_sqlalchemy_engine()` had not been implemented in PostgresHook.

When using `PostgresHook('CONNECTION_NAME', schema='another_schema').get_sqlalchemy_engine()` that will still use connection default schema setting through `get_uri()`, instead of schema that is assigned to `PostgresHook()`.

**What you expected to happen**:
`get_uri()` should follow schema in PostgresHook.

**How to reproduce it**:
`PostgresHook('CONNECTION_NAME', schema='another_schema'). get_uri()`

**Anything else we need to know**:


airflow/providers/postgres/hooks/postgres.py
"""
        Uses AWSHook to retrieve a temporary password to connect to Postgres
        or Redshift. Port is required. If none is provided, default is used for
        each service
        """
        from airflow.providers.amazon.aws.hooks.base_aws import AwsBaseHook
        redshift = conn.extra_dejson.get('redshift', False)
        aws_conn_id = conn.extra_dejson.get('aws_conn_id', 'aws_default')
        aws_hook = AwsBaseHook(aws_conn_id, client_type='rds')
        login = conn.login
        if conn.port is None:
            port = 5439 if redshift else 5432
        else:
            port = conn.port
        if redshift:
            
            
            cluster_identifier = conn.extra_dejson.get('cluster-identifier', conn.host.split('.')[0])