Function provider_fn 

pub fn provider_fn<F>(f: F) -> FnProvider<F>
where
    F: Fn(&CredentialRequest<'_>) -> Result<Option<Credential>, Error> + Send + Sync,

Adapt a synchronous closure into a CredentialProvider. The closure runs at request time and returns the credential (or None to defer to ambient auth). For async sources (a network vault), implement CredentialProvider directly.