Make Helenus Pekko a part of Core

[nMoncho]

Problem

We need to treat Akka in a special way if we want to support both the Apache License and BUSL. But we don't need to do this with Pekko, we can simply provide it as an optional transitive dependency. This will simplify user adoption and developer maintenance.

Solution Description

• Helenus Pekko is no longer provided as an extra module and is included in core
• Helenus can still work without users importing Pekko (e.g. with Akka)

[nMoncho]

Won't implement