RNTuple: add max key length field to RNTuple anchor

[silverweed]

Explain what you would like to see improved and how.

(This description is work in progress)

The TKey payload size is limited to a few (1?) GB, which may be too little to store all the required header data of a RNTuple.
The proposed solution is to add an explicit maxKeySize field to the RNTuple anchor which encodes this limit and, when a blob is read which exceeds this limit, to interpret it as a list of pointers to other blobs.

ROOT version

6.33/01

Installation method

source

Operating system

Linux

Additional context

No response