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

[jblomer]

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