Implements /transactions PUT method

Adds PUT method to OpenAPI spec.

Given that the transaction IDs are generated on server-side, for the PUT
method to remain idempotent, it can only update existing transactions.

It also adds a TransactionExists method on the DAL.

Issue: #20

internal/dal/impl_test.go

@@ -355,7 +355,7 @@ func TestDalImpl_UpdateTransaction(t *testing.T) {
 			mock.
 				ExpectExec(`
 					UPDATE pfbudget.transactions
-					SET date = \$2, description = \$3, value = \$4
+					SET date = \$2, description = \$3, amount = \$4
 					WHERE id = \$1`).
 				WithArgs(tt.args.t.Id, tt.args.t.Date, tt.args.t.Description, tt.args.t.Value).
 				WillReturnResult(sqlmock.NewResult(0, 1))