fix(secretsmanager): fix cross-region policy arn for imported secrets

[rv2673]

Pre 2.89.0 we could import a secret from a complete secret arn in one stack and reference this secret from another stack in a different region to include it in a policy/role through grantRead on the secret construct.

Since 2.89.0 the arn in the policy it treats the compledSecretArn as a partial arn adding -?????? which makes the policy invalid and not allowing access to the secret as intended.

This PR fixes that by overriding arnForPolicies for imported secrets to either return provided complete arn or partial arn with suffix.

Fixes #26811.

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