Add a relation for fetching transactions from the wallet (walletTransactions)

[rez1dent3]

#416

// default wallet
$user->deposit(100);
$user->wallet->deposit(200);
$user->wallet->withdraw(1);

// usd
$usd = $user->createWallet(['name' => 'USD']);
$usd->deposit(100);

// eur
$eur = $user->createWallet(['name' => 'EUR']);
$eur->deposit(100);

$user->transactions()->count(); // 5
$user->wallet->transactions()->count(); // 5
$usd->transactions()->count(); // 5
$eur->transactions()->count(); // 5
// the transactions method returns data relative to the owner of the wallet, for all transactions

$user->walletTransactions()->count(); // 3. we get the default wallet
$user->wallet->walletTransactions()->count(); // 3
$usd->walletTransactions()->count(); // 1
$eur->walletTransactions()->count(); // 1