Add support for role chaining

The idea is to be able to use secret manager from different account.

e.g. caching script running in account A and fetching secrets from account B using cross account role.

What do you think?

Thanks

The issue describes a Python caching client fetching secrets from account B through a cross-account role while running in account A. Start by reviewing the repository's current AWS Secrets Manager and credential behavior, then clarify the role-chaining design and its security requirements. Done means the supported cross-account flow is defined and demonstrated with appropriate tests.

Written by the indexing model from the issue text.