**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
"""
        Runs a command or a list of commands. Pass a list of sql
        statements to the sql parameter to get them to execute
        sequentially
        :param sql: the sql statement to be executed (str) or a list of
            sql statements to execute
        :type sql: str or list
        :param autocommit: What to set the connection's autocommit setting to
            before executing the query.
        :type autocommit: bool
        :param parameters: The parameters to render the SQL query with.
        :type parameters: dict or iterable
        :param handler: The result handler which is called with the result of each statement.
        :type handler: callable
        :return: query results if handler was provided.
        """
        scalar = isinstance(sql, str)
        if scalar:
            sql = [sql]