Have the Shutdown and Init functions take a context.Context parameter and return an error.

`Shutdown` and `Initialize` provider functions must accept a context.More specifically, new versions of them should accept a `context.Context` and return an error, while keeping the old versions so as not to create a breaking change for providers.

The global API `Shutdown` function should also accept a context, which is passed to the new provider `Shutdown` functions mentioned above. This change is breaking; there will be no `context-less` `Shutdown` variant on the global API object.