support multi-statement `.execute()` calls

## ask

related to: dbt-labs/dbt-core#39 dbt-labs/dbt-core#46 dbt-labs/dbt-core#82

allow for dbt (and other consumers/users) to make use of this driver in a simple query flow manner. two specific places that introduce friction are:
1. strings with multiple, `;`-delimited SQL statements cannot be sent to `cursor.execute()
2. by default the driver today interprets every SQL operation to be a prepared statement even when it isn't.

I love @Brooke-white 's summary in https://github.com/aws/amazon-redshift-python-driver/issues/39#issuecomment-915393795. I've been looking for this terminology all week!

> yes I agree it'd be much cleaner (and simpler) to support this in a single call to execute.
> I believe `psycopg` supports this due to the approach taken to executing the statement. There are two approaches a driver can take to execute a statement -- [simple query cycle](https://www.postgresql.org/docs/11/protocol-flow.html#id-1.10.5.7.4) and[ extended query cycle](https://www.postgresql.org/docs/11/protocol-flow.html#PROTOCOL-FLOW-EXT-QUERY). Simple query cycle supports executing multiple statements, while extended query protocol does not. `redshift_connector` uses extended query protocol, which is the root cause for our lack of support. If you're interested, the above links give an interesting overview of the two approaches and their trade offs.
> When I have extra time I am working to add support for simple query cycle (i.e. multiple statement execution), but there's no timeline on when this would be available as we are prioritizing other tasks at this time. Ideally, I'd like to provide support for both extended and simple query cycles so users have the option to fine tune how statements are executed :)

## background

dbt is a framework that effectively takes `SELECT` statements, and wraps them within boilerplate DDL to create a DAG of tables and views. Only a somewhat auxillary feature, [seeds](https://docs.getdbt.com/docs/build/seeds), make use of prepared statements with an `INSERT INTO ... VALUES` statement.

In some ways, dbt is a stand in for the parsing and preparation that a database might do in an extended query flow pattern. Using both both complicates things greatly.

recently, in https://github.com/dbt-labs/dbt-redshift/pull/251, dbt-redshift migrated away from `psycopg2` to `redshift_connector` in order to enable
1. IAM auth to redshift serverless, and
2. the redshift team to more proactively accommodate and fix issues upstream.

however this change has introduced a number of regressions for our users. While we have both:
- https://github.com/dbt-labs/dbt-redshift/pull/475 a workaround that forces `autocommit=True` for all connections, as well as
- https://github.com/dbt-labs/dbt-adapters/issues/347 a backlog item to revisit query execution and transactions more generally across all our supported data platforms

## example
```python
cursor = redshift_connector.connect(**vals).cursor()
cursor.execute("select 1 as id; select 2 as id")
records_rs = cursor.fetchall()
```
Error message ([full traceback](https://gist.github.com/dataders/9581366ecf8b2bff4e6a1988164f593d))
```
ProgrammingError: {'S': 'ERROR', 'C': '42601', 'M':
'cannot insert multiple commands into a prepared statement',
'F': '../src/pg/src/backend/tcop/postgres.c','L': '3916', 'R': 'exec_parse_message'}
```

## counter-counter-argument

@Brooke-white I saw you mention somewhere that [the spec for .execute() in the PEP 249 – Python Database API Specification v2.0
](https://peps.python.org/pep-0249/#id20) lays out that the input may only be a single `operation` and that `.executemany()` should be used for multi-operation/-statement queries. However, if I'm reading this right, the purpose of `.executemany()` is to execute the the operation over a list of parameter sets. The first argument is still a singular `operation`.

## sidebar

I'm very unclear on exact meanings of the following terms here: `query`, `statement`, `operation`

## further complications

all of this confusion is making it even harder to determine how transactions are handled by default between the simple and extended query flows

cursor.execute() のフローと、セミコロンで区切られたステートメントを使用する例から始めてください。リンクされている PostgreSQL simple and extended query protocol のドキュメントを読み、複数ステートメントの実行とトランザクションの動作がどうあるべきかを判断してください。完了条件は、要求されたクエリフローが prepared-statement エラーなしでサポートされることです。

索引モデルが issue の本文から書いたものです。