fix: hide anonymous RNTuple fields by default

[ariostas]

This PR hides the anonymous fields present in RNTuples. I made this a draft because I want to spend some more time thinking about this to make sure everything that people might want to access is actually easily accessible.

This is a rough first draft that has some downsides. For example, if you have a tuple field, the underlying data in each position of the tuple is stored in fields _0, _1, ..., so those would be hidden, but those should actually be accessible. Similarly, if you have an array of structs, let's say an array of electrons, then the members get stored in _0.pt, _0.eta, ..., so again those would be hidden, but should actually be accessible.

The tricky part that I'll have to figure out is how to separate "key paths" from the "real paths" inside the RNTuple (e.g. electron.pt vs electron._0.pt).

Thanks to @kratsg for bringing up this issue.