FR: Add option for hash input order when importing users

#### Feature Request Description

Add a new `hash.inputOrder` option for the [`importUsers()`](https://firebase.google.com/docs/reference/admin/node/admin.auth.Auth#importUsers) auth method, inspired by the [`--hash-input-order` parameter of the Firebase CLI](https://firebase.google.com/docs/cli/auth).

It will accept a string value of `"SALT_FIRST"` or `"PASSWORD_FIRST"` and it will only apply to the `SHA512`, `SHA256`, `SHA1`, `MD5`, `HMAC_SHA512`, `HMAC_SHA256`, `HMAC_SHA1`, and `HMAC_MD5` algorithms.

#### Usage Example

```js
const importOptions = {
hash: {
algorithm: 'SHA256',
rounds: 80,
inputOrder: 'PASSWORD_FIRST', // or 'SALT_FIRST'
}
};

return admin.auth().importUsers(users, importOptions)
.then((results) => {
console.log('Users successfully imported!', results);
})
.catch((error) => {
console.log('Failed to import users:', error);
});
```