[FEAT] Provide the ability to set isolation level using the `@Transactional` decorator
## Information

As a user, I would like to set the transaction isolation levels using `@Transactional` decorator,  defining the type of locks acquired on DB operations.

The new option can be introduced, extending the `TransactionOptions` as follows:

```ts
export interface TransactionOptions {
  retry?: boolean;
  isolationLevel?: IsolationLevel;
  contextName?: string;
}
```

Where `IsolationLevel` is enum from the `@mikro-orm/core` package:

```ts
export declare enum IsolationLevel {
    READ_UNCOMMITTED = "read uncommitted",
    READ_COMMITTED = "read committed",
    SNAPSHOT = "snapshot",
    REPEATABLE_READ = "repeatable read",
    SERIALIZABLE = "serializable"
}
```


## Acceptance criteria

- [ ] The isolation level can be set via `@Transactional` decorator in this way: `@Transactional({ isolationLevel: IsolationLevel.REPEATABLE_READ })`
- [ ] `IsolationLevel.READ_COMMITTED` is an isolation level by default 
