The idea is to use the `Subscriptions` struct from Substrate, which
is used to drive subscription futures to completion, and modify
it for "general" use.

Adds trait bounds that allow conversion between the two, removing
the need for generics in SubscriptionId.

Adds two subscription ID providers which can be used
by the SubscriptionManager. One provides a simple numeric
ID, while the other provides a random string.

Instead of providing a guarantee that we can convert between
`usize` and `u64` we make the assumptions that it's unlikely
that we're running on an architecture larger than 64-bits and
we use a `u64` directly.

Note: There's one test that doesn't pass yet which has to do
with the `cancel()` function of the SubscriptionManager.

Our Windows CI runner isn't up to date and thinks this
is still a nightly feature