catalyst-api/catalyst: use common stream name PREFIX for detecting live vs VOD streams.

[emranemran]

We currently have prefixes defined here: https://github.com/livepeer/catalyst-api/blob/main/config/stream_names.go#L12

But we need to also use these prefixes in catalyst-node. For example: https://github.com/livepeer/catalyst/pull/177

We should have a common source of truth so that we're not modifying code in two places for every change in PREFIX names.