sslmode does not support allow, prefer, require, disable

## Driver version

2.0.910

## Redshift version

PostgreSQL 8.0.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.4.2 20041017 (Red Hat 3.4.2-6.fc3), Redshift 1.0.49780

## Client Operating System

macos monterey 12.6.2

## Python version

3.11
## Table schema

Does not apply
## Problem description

1. Expected behaviour:
In [postgreSQL](https://www.postgresql.org/docs/current/libpq-ssl.html#:~:text=SSL%20Mode%20Descriptions-,sslmode,-Eavesdropping%20protection), these 5 parameters are allowed values for sslmode. However, redshift_connector only allows for verify-ca, and verify-full for this parameter. Redshift_connector also has ssl as a parameter.

2. Actual behaviour:
There are a few problems with this difference between postgreSQL and redshift_connector:
a. To disable ssl, users using redshift_connector has to set ssl = False. Simply setting sslmode = disable will not set ssl to false. Since disable is not a recognizable value of sslmode in redshift_connector, redshift_connector will use the default of 'verify-ca' to make the connection.
b. According to the PostgreSQL [doc](https://www.postgresql.org/docs/current/libpq-connect.html#:~:text=%2Dencrypted%20connection.-,sslmode,-This%20option%20determines), the accepted values of sslmode behave as below:

disable
only try a non-SSL connection

allow
first try a non-SSL connection; if that fails, try an SSL connection

prefer (default)
first try an SSL connection; if that fails, try a non-SSL connection

require
only try an SSL connection. If a root CA file is present, verify the certificate in the same way as if verify-ca was specified

verify-ca
only try an SSL connection, and verify that the server certificate is issued by a trusted certificate authority (CA)

verify-full
only try an SSL connection, verify that the server certificate is issued by a trusted CA and that the requested server host name matches that in the certificate

Redshift_connector should also increase the values accepted by sslmode to align with PostgreSQL docs

After some investigation, here is a detailed table on the behavior of sslmode of redshift_connector and psycopg2:

sslmode | behavior in redshift connector (ssl, sslmode) | behavior in psycopg2 connector (sslmode)
-- | -- | --
disable | ssl=defaulted to true, sslmode=verify-ca (sslmode of disable is not recognized by redshift_connector, therefore falling back to default of verify-ca) | sslmode=disable
allow | ssl=defaulted to true, sslmode=verify-ca | first try with sslmode=disable, if fails, try with sslmode=verify-ca
prefer | ssl=defaulted to true, sslmode=verify-ca | first try with sslmode=verify-ca, if fails, try with sslmode=disable
require | ssl=defaulted to true, sslmode=verify-ca | ssl=true, sslmode=verify-ca
verify-ca | ssl=defaulted to true, sslmode=verify-ca | ssl=true, sslmode=verify-ca
verify-full | ssl=defaulted to true, sslmode=verify-full | ssl=true, sslmode=verify-full

Locate the sslmode validation and connection setup in redshift_connector, then compare the current behavior with the PostgreSQL sslmode documentation and the behavior table in this issue. Add coverage for the six listed values and verify that each mode produces the documented connection behavior.

Written by the indexing model from the issue text.