Support hostname verification to allow `verify-full` SSL/TLS connections

[hessjcg]

Feature Description

The Cloud SQL Python Connector currently only supports verify-ca
as it's highest level of SSL/TLS client verification.

verify-ca: verify that the server is trustworthy by checking the certificate chain up to the root certificate

We are only getting verify-ca level of security due to hostname verification
being disabled currently .

cloud-sql-python-connector/google/cloud/sql/connector/connection_info.py

Lines 60 to 61 in 53e40c7

	# update ssl.PROTOCOL_TLS_CLIENT default
	context.check_hostname = False

We should support verify-full as it is the highest level of security.

verify-full: will also verify that the server host name matches the name stored in the server certificate

To do so we must support hostname verification.

For Public IP and PSA Private IP connections, the server certificate's
subject CN field in the server certificate will contain the instance name
in the form "{project}:{instance}". The python connector should validate
that this is correctly set during the TLS handshake. The connector should
reject the connection if this the server CN is not set correctly.

Unfortunately for PSC enabled and CAS-based instances hostname
verification is not possible. This is due to the server certificates including
the DNS name as a SAN which include a trailing dot ("."). Python's SSL does
not allow trailing dots during hostname verification, it requires stripping them.
This leads to hostname mismatch errors as the SAN is expecting a trailing dot.

See also: GoogleCloudPlatform/cloud-sql-jdbc-socket-factory#1995

[jackwotherspoon]

This is going to be slightly complex to validate during the TLS handshake.

For pg8000, pymysql and pytds it is easy as we can change the server_hostname to project:instance for old instances and to the DNS name for CAS and PSC (barring the SAN issue gets fixed).

But for asyncpg we don't have control over the handshake at all as it is done in the asyncpg code, we just pass it the SSL/TLS context object.

cloud-sql-python-connector/google/cloud/sql/connector/asyncpg.py

Lines 55 to 64 in de2852f

	return await asyncpg.connect(
	user=user,
	database=db,
	password=passwd,
	host=ip_address,
	port=SERVER_PROXY_PORT,
	ssl=ctx,
	direct_tls=True,
	**kwargs,
	)

[jackwotherspoon]

Adding context link for DNS name with trailing dot: openssl/openssl#11560

[jackwotherspoon]

Will need to make sure we have this cover CAS based Ca's where the DNS name should be set as the server hostname to verify, will be CUSTOMER_MANAGED_CAS_CA and GOOGLE_MANAGED_CAS_CA