[FormField] Add option to allow hint to be shown only when focused

#### Bug, feature request, or proposal:
Feature request

#### What is the expected behavior?
When the option is specified, the hint/helper text is hidden unless the field has focus.

#### What is the current behavior?
The hint is always displayed unless no hint is used

#### What is the use-case or motivation for changing an existing behavior?
Material Design specifications call for the hint to be either always displayed or displayed on focus only: https://material.io/guidelines/components/text-fields.html#text-fields-layout (see 'Helper text').

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Material v5

Inizia dall’implementazione di Angular Material FormField e segui come viene attualmente eseguito il rendering del testo guida/di aiuto. Conferma gli stati atteso con focus e senza focus dell’opzione, quindi verifica che il suggerimento sia nascosto senza focus e rimanga visibile quando ha il focus.

Scritto dal modello di indicizzazione a partire dal testo della issue.