Allow generating a custom profile of question types

[ceberam]

The default Q&A generation tries to generate 1 question of each type for every chunk.

Some use cases may require more questions of a specific type (e.g., summary vs single value). Even though the prompt can be passed as an option in GenerateOptions object, there is no flexibility in terms of:

• number of target questions per passage (chunk)
• percentage of question per type

This enhancement should allow users have a more customized Q&A dataset