Spike: Define Scaler trait

[thomastaylor312]

A scaler will need to take a config of some kind, likely a handle to some sort of state, and two different methods. Below is a general idea of what it could look like, but part of this spike is to have an integration test that can pass in an Event or a wadm manifest and output commands with a basic, not fully functional spreadscaler.

/// A Scaler is used to manage responding to even
pub trait Scaler {
  /// Any type that has the necessary data to configure the scaler
  type Config: Send + Sync;

  /// Handles the event, returning any needed changes in response
  fn handle_event(&self, event: ScopedEvent) -> Result<HashSet<Command>>;

  /// Handles a new or updated manifest with its given config. This configuration should be stored by implementors in some form so that handle_event can produce the right commands
  fn handle_manifest(&self, config: Self::Config) -> Result<HashSet<Command>>;

  /// Removes a config from the scaler and emits the expected compensatory commands
  fn remove_manifest(&self, config: &Self::Config) -> Result<HashSet<Command>>;
}

[thomastaylor312]

Closed in #71