filemanager: replace mockall with aws-smithy-mocks-experimental

[mmalenic]

The mockall crate is able to mock AWS clients for tests, but since there is now an AWS-native solution, it would probably be better to use that. The aws-smithy-mocks-experimental crate is already being used for the S3 inventory integration tests, and it seems to work well. It could also be considered for the other unit tests.