feat: add randomString method to RandomService

[magonxesp]

Add some methods for generate a random string as alternative to lorem ipsum for generate strings with specific characters or alphanumeric characters.

[bodiam]

Please have a look here, we've got quite a few text generation options in Datafaker:

https://github.com/datafaker-net/datafaker

Also, we're happy to take pull requests if something is missing!