Improve @kbn/config-schema string type with generics

[dimaanj]

Working with @kbn/config-schema were found that it would be nice to have generic typing for string schema type. Since in some cases it might be enum like here is Comparator. In that case it will be possible to get rid of type casting (as). Suggestion:

function string<T extends string>(options?: StringOptions<T>): Type<T>;

type StringOptions<T extends string> = TypeOptions<T> & {
    minLength?: number;
    maxLength?: number;
    hostname?: boolean;
};

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[afharo]

@dmitriynj have you considered using schema.oneOf for this type of validation?

  thresholdComparator: schema.oneOf([
    schema.literal(Comparator.GT),
    schema.literal(Comparator.LT),
    schema.literal(Comparator.GT_OR_EQ),
    schema.literal(Comparator.LT_OR_EQ),
    schema.literal(Comparator.BETWEEN),
    schema.literal(Comparator.NOT_BETWEEN),
  ]),

[dimaanj]

@dmitriynj have you considered using schema.oneOf for this type of validation?

schema.oneOf solves the problem with type casting for enums, thank you. Therefore closing it.