**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**:


tests/providers/postgres/hooks/test_postgres.py
def setUp(self):
        super().setUp()
        self.connection = Connection(login='login', password='password', host='host', schema='schema')
        class UnitTestPostgresHook(PostgresHook):
            conn_name_attr = 'test_conn_id'
        self.db_hook = UnitTestPostgresHook()
        self.db_hook.get_connection = mock.Mock()
        self.db_hook.get_connection.return_value = self.connection
    @mock.patch('airflow.providers.postgres.hooks.postgres.psycopg2.connect')
    def test_get_conn_non_default_id(self, mock_connect):
        self.db_hook.test_conn_id = 'non_default'  
        self.db_hook.get_conn()
        mock_connect.assert_called_once_with(
            user='login', password='password', host='host', dbname='schema', port=None
        )
        self.db_hook.get_connection.assert_called_once_with('non_default')
    @mock.patch('airflow.providers.postgres.hooks.postgres.psycopg2.connect')
    def test_get_conn(self, mock_connect):
        self.db_hook.get_conn()
        mock_connect.assert_called_once_with(
            user='login', password='password', host='host', dbname='schema', port=None
        )
    @mock.patch('airflow.providers.postgres.hooks.postgres.psycopg2.connect')
    def test_get_conn_cursor(self, mock_connect):