Support for multiple subscriptions on the same Consumer

[svroonland]

With this PR, a single Consumer instance and its backing apache kafka Consumer instance, can support multiple topic/pattern/manual subscriptions (as long as they are of the same type).

Inspired by https://doc.akka.io/docs/alpakka-kafka/current/consumer.html#sharing-the-kafkaconsumer-instance

• Subscriptions are now passed as parameter to plainStream/partitionedStream/partitionedAssignmentStream.
• Removed subscribeAnd, SubscribedConsumer, SubscribedConsumerFromEnvironment.
• Subscriptions are scoped to the lifetime of the plainStream/partitionedStream/partitionedAssignmentStream.
• subscribe and unsubscribe have been removed from the Consumer trait in favour of scoped subscriptions.
• When an error occurs downstream of plainStream/partitionedStream, the stream may be restarted thanks to the scoped subscriptions. This was not possible previously, requiring to make a new Consumer instance (or perhaps unsubscribe & resubscribe).

Implements #217.

[svroonland]

@vigoo @adamgfraser @guizmaii Could I get your opinion on this interface change?

[guizmaii]

@svroonland Can you rebase your branch, please?

[svroonland]

The only merge conflict is in some docs. Let's gather some feedback on these changes first before merging.

[nmcb]

by lack of understanding - given the description of this pr - and difference between: restartable plainStream, partitionedStream and non restartable partitionedAssignmentStream i would expect the type signatures and/or their documentation to differ more prominently - am i missing something ?

[guizmaii]

I tried to find a way to be more typesafe: no more runtime error possible with the "union" operation, but I didn't find any way because in the end you're just composing streams 🤔

[guizmaii]

Here's a proposal to improve a bit the readability: #663

Can you rebase your branch also, please :)