**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
"""
    Interact with Postgres.
    You can specify ssl parameters in the extra field of your connection
    as ``{"sslmode": "require", "sslcert": "/path/to/cert.pem", etc}``.
    Also you can choose cursor as ``{"cursor": "dictcursor"}``. Refer to the
    psycopg2.extras for more details.
    Note: For Redshift, use keepalives_idle in the extra connection parameters
    and set it to less than 300 seconds.
    Note: For AWS IAM authentication, use iam in the extra connection parameters
    and set it to true. Leave the password field empty. This will use the
    "aws_default" connection to get the temporary token unless you override
    in extras.
    extras example: ``{"iam":true, "aws_conn_id":"my_aws_conn"}``
    For Redshift, also use redshift in the extra connection parameters and
    set it to true. The cluster-identifier is extracted from the beginning of
    the host field, so is optional. It can however be overridden in the extra field.
    extras example: ``{"iam":true, "redshift":true, "cluster-identifier": "my_cluster_id"}``
    :param postgres_conn_id: The :ref:`postgres conn id <howto/connection:postgres>`
        reference to a specific postgres database.
    :type postgres_conn_id: str
    """
    conn_name_attr = 'postgres_conn_id'
    default_conn_name = 'postgres_default'
    conn_type = 'postgres'
    hook_name = 'Postgres'
    supports_autocommit = True
    def __init__(self, *args, **kwargs) -> None: