Add missing types to FieldSet interface.

[przemekciacka]

Why

When you have lookup field in the table that refers to the column of type number or boolean, it's impossible to define its type as number[] nor boolean[]. In such situation the following compiler's error is reported.

Type 'ServiceFields' does not satisfy the constraint 'FieldSet'.
  Property 'price' is incompatible with index signature.
    Type 'number[]' is not assignable to type 'string | number | boolean | Collaborator | readonly Collaborator[] | readonly string[] | readonly Attachment[] | undefined'.
      Type 'number[]' is not assignable to type 'readonly Attachment[]'.
        Type 'number' is not assignable to type 'Attachment'.

You can always defined such fields as string[]. However, this requires to cast types later in the code. For example, when field is numeric, the casting looks like the following:

fields.price[0] as unknown as number

This works, but that's not be best practice using TypeScript.

What

This PR resolves above issue by extending FieldSet interface by ReadonlyArray<number> and ReadonlyArray<boolean> types.

[jamescrowley]

This would be great to get fixed...

There's also the somewhat weird case that when this lookup field is a boolean and it's set to false/unchecked, it actually comes back as null in the API instead of false

[albertlieyingadrian]

Also facing the same issue and need this proposal to be merged

[albertlieyingadrian]

LGTM