Add a way to limit the length of the generated string in the DataProviderStrategy

[mipo256]

I'm using PODAM to fill POJOs with the random data before persistence to the storage. Some fields are mapped to columns of the VARCHAR(N) type, which means that they are limited in length.

When researching the AbstractRandomDataProviderStrategy, I have not found any way to:

1. Set the max generated String length for the given object for all String fields
2. Set the max generated String length for the particular fields of the pojo

So, adding such an ability would be great.