**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/hooks/dbapi.py
"""Sets the autocommit flag on the connection"""
        if not self.supports_autocommit and autocommit:
            self.log.warning(
                "%s connection doesn't support autocommit but autocommit activated.",
                getattr(self, self.conn_name_attr),
            )
        conn.autocommit = autocommit
    def get_autocommit(self, conn):
        """
        Get autocommit setting for the provided connection.
        Return True if conn.autocommit is set to True.
        Return False if conn.autocommit is not set or set to False or conn
        does not support autocommit.
        :param conn: Connection to get autocommit setting from.
        :type conn: connection object.
        :return: connection autocommit setting.
        :rtype: bool
        """
        return getattr(conn, 'autocommit', False) and self.supports_autocommit
    def get_cursor(self):
        """Returns a cursor"""
        return self.get_conn().cursor()
    @staticmethod
    def _generate_insert_sql(table, values, target_fields, replace, **kwargs):