Fix _create_session_config isolation

[yaroslav-ost]

This PR addresses the issue related to sharing _create_session_config class attribute among Glue sessions. The fix prevents from overriding this attribute and enhances isolation.

Relates to https://github.com/aws-samples/dbt-glue/pull/234

Description

Checklist

• [ ] I have signed the CLA
• [x] I have run this code in development and it appears to resolve the stated issue
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] I have updated the CHANGELOG.md and added information about my change to the "dbt-glue next" section.

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[yaroslav-ost]

@menuetb pls review and merge

[moomindani]

Thank you for your contribution!