chore(cfn): add DescribeTable permission for legacy DDBEC support

[imabhichow]

Issue #, if available:

Description of changes:

• This allows the DB-ESDK for Python to test the legacy extern behavior.
• See: https://docs.aws.amazon.com/database-encryption-sdk/latest/devguide/python-using.html#python-helpers
• To use the client helper classes in DDBEC, the caller must have permission to call the DynamoDB DescribeTable operation on the target table.

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[texastony]

This only grants Describe Table permission for the DynamoDB table used by the DB-ESDK's Interceptor.

Your PR overview says you need it for the legacy table.
Just add the action to the previous statement that covers all tables; that should cover both the legacy and contemporary tables.

[imabhichow]

CFN Changeset deployed successfully