[ResponseOps][Alerting] Rule and action's form errors are not typed correctly

[cnasikas]

The alerting framework uses and passes an error object to the rule's and connector's params component to be able the components to show errors and disable their form inputs. The error object is typed as:

export interface IErrorObject {
  [key: string]: string | string[] | IErrorObject;
}

export interface RuleFormParamsErrors {
  [key: string]: string | string[] | RuleFormParamsErrors;
}

This leads to a lot of casting in the code because each component uses only one schema for the errors or the framework does not handle all error schemas. We should have one common error schema across all rule and connector forms. For example:

export interface Error {
  [key: string]: string;
}

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)