Sendable conformance for `Container`

[erichoracek]

With Swift 5.10 released and Swift 6 on the horizon, consumers of Factory would expect Container to conform to Sendable as it is intended to be thread safe. However, since it contains a property of containerManager, it's not possible to mark it as such:

Furthermore, when you attempt to conform ContainerManger to sendable, you notice that it has publicly writable properties that are not protected, namely defaultScope but also some other debug ones, so it cannot conform to Sendable either:

To satisfy the Sendable contract, ideally Container and ContainerManager would both have Sendable conformances, though this might require an API rethink.