[Alerting] Make full Alert<Params> object accessible in Alert executors

[marshallmain]

Describe the feature:
Currently the Alert executor options include

  alertId: string;
  startedAt: Date;
  previousStartedAt: Date | null;
  services: AlertServices<InstanceState, InstanceContext, ActionGroupIds>;
  params: Params;
  state: State;
  spaceId: string;
  namespace?: string;
  name: string;
  tags: string[];
  createdBy: string | null;
  updatedBy: string | null;

however the full Alert object is not directly accessible in the executor. Thus the Alert fields actions, createdAt, updatedAt, and schedule.interval can't be retrieved without using the saved objects client to get the raw alert SO. However, using the saved objects client like this exposes executor functions to more internals of how the Alerting framework is implemented. It would be useful to pass in Alert<Params> rather than just the Params and safer than using the SO client.

Describe a specific use case for the feature:
In the security solution we record the state of the Alert at the time of execution in all signal documents that we write to .siem-signals.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)