**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/hooks/test_dbapi.py
table = "table"
        rows = [("hello",), ("world",)]
        commit_every = 1
        self.db_hook.insert_rows(table, rows, commit_every=commit_every)
        assert self.conn.close.call_count == 1
        assert self.cur.close.call_count == 1
        commit_count = 2 + divmod(len(rows), commit_every)[0]
        assert commit_count == self.conn.commit.call_count
        sql = f"INSERT INTO {table}  VALUES (%s)"
        for row in rows:
            self.cur.execute.assert_any_call(sql, row)
    def test_get_uri_schema_not_none(self):
        self.db_hook.get_connection = mock.MagicMock(
            return_value=Connection(
                conn_type="conn_type",
                host="host",
                login="login",
                password="password",
                schema="schema",
                port=1,
            )
        )
        assert "conn_type://login:password@host:1/schema" == self.db_hook.get_uri()
    def test_get_uri_schema_none(self):