…d object (elastic#75553)

resolves elastic#51099

This formalizes the concept of "alert status", in terms of it's execution, with
some new fields in the alert saved object and types used with the alert client
and http APIs.

These fields are read-only from the client point-of-view; they are provided in
the alert structures, but are only updated by the alerting framework itself.
The values will be updated after each run of the alert type executor.

The data is added to the alert as the `executionStatus` field, with the
following shape:

```ts
interface AlertExecutionStatus {
  status: 'ok' | 'active' | 'error' | 'pending' | 'unknown';
  lastExecutionDate: Date;
  error?: {
    reason: 'read' | 'decrypt' | 'execute' | 'unknown';
    message: string;
  };
}
```