Convenience methods for constructing a custom IORuntime

[etspaceman]

I recently had the need to manually construct an IORuntime, which took in an Executor and used it as its compute pool. Doing this was... not the most intuitive thing in the world.

def executorRuntime(EC: ExecutionContext): IORuntime {
  override val runtime: IORuntime = {
    val (blocking, _) = IORuntime.createDefaultBlockingExecutionContext()
    val (scheduler, _) = IORuntime.createDefaultScheduler()

    IORuntime(EC, blocking, scheduler, () => (), IORuntimeConfig())
  }
}

It would be interesting to have some of these convenience methods in cats effect. Or, at the very least, some documentation on doing these things.

I opened this as a discussion per a conversation with @djspiewak, as we may not want to encourage these operations (since the default IORuntime has a lot of benefits).