[ntuple] Better customization options for RClusterPool (read-ahead)

### Explain what you would like to see improved and how.

The RClusterPool should be improved
- to work with a memory budget rather than a fixed number of clusters to read-ahead
- allow users to exclude/include specific fields

### ROOT version

master

### Installation method

n/a

### Operating system

n/a

### Additional context

_No response_