Fix typing of kerberos mutual_authentication

[e1senh0rn]

Description

Fix #513 by correcting mutual_authentication type annotation and adding corresponding mutual authentication modes constants.

• Introduce constants for Kerberos and GSSAPI mutual authentication modes within KerberosAuthentication and GSSAPIAuthentication classes.
• Set proper type annotations and default values for mutual_authentication attributes.
  • Default values are replicated from corresponding defaults in requests_kerberos/requests_gssapi.

Non-technical explanation

Kerberos/GSSAPI libraries (requests_kerberos and requests_gssapi) expect mutual_authentication parameter to be an integer:

• 1 for "required"
• 2 for "optional"
• 3 for "disabled"

However, the same parameter in trino-python-client is annotated as bool. By luck, due to the order of conditions and the fact that True == 1, Kerberos/GSSAPI libraries handle True/False as "required"/"disabled" modes.

This is non-breaking change, but it can streamline the usage of KerberosAuthentication a bit:

### before
from requests_kerberos import OPTIONAL
from trino.auth import KerberosAuthentication

auth = KerberosAuthentication(
    principal="username",
    mutual_authentication=OPTIONAL,  # type: ignore
    service_name="HTTP",
)

### after
from trino.auth import KerberosAuthentication

auth = KerberosAuthentication(
    principal="username",
    mutual_authentication=KerberosAuthentication.OPTIONAL,
    service_name="HTTP",
)

Release notes

( ) This is not user-visible or docs only and no release notes are required.
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(x) Release notes are required, with the following suggested text:

* Fix type annotation for `mutual_authentication` parameter in Kerberos/GSSAPI authentication ({issue}`513`).

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