[Hook form lib] FormSchema does not narrow the index signature

[sebelga]

As reported by @rylnd in #60567, the FormSchema interface does not narrow the index signature.

The changes he proposes are

-export interface FormSchema<T extends FormData = FormData> {
-  [key: string]: FormSchemaEntry<T>;
-}
+export type FormSchema<T extends FormData = FormData> = {
+  [K in keyof T]: FormSchemaEntry<T[K]>;
+};

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)