Add upsert support

[piercefreeman]

In Postgres, an upsert is just a special case of INSERT with the extended ON CONFLICT flags. While we could override our connection.insert method to support this case, it overly complicates the API contract for a case that is pretty disjoint in practical usage. This PR adds new explicit logic to insert a batch of objects and optionally return fields:

    await db_connection.conn.execute(
        """
        ALTER TABLE userdemo
        ADD CONSTRAINT email_unique UNIQUE (name, email)
        """
    )

    users = [
        UserDemo(name="John Doe", email="[email protected]"),
        UserDemo(name="John Doe", email="[email protected]"),
        UserDemo(name="Jane Doe", email="[email protected]"),
    ]
    result = await db_connection.upsert(
        users,
        conflict_fields=(UserDemo.name, UserDemo.email),
        returning_fields=(UserDemo.name, UserDemo.email),
    )

    assert result is not None
    assert len(result) == 2
    assert {r[1] for r in result} == {"[email protected]", "[email protected]"}