Consider renaming namespace ID to namespace

[rootulp]

Context

This library is used in celestia-app. After celestia-app/adr-014 and celestia-app/adr-015 were introduced, the namespaced data that gets pushed to an NMT consists of: namespace version (1 byte) | namespace ID (28 bytes) = namespace (29 bytes).

Problem

It is a bit confusing that this repo refers to them as namespace IDs when celestia-app refers to them as namespaces.

Proposal

Rename namespace ID to namespace in this repo

[staheri14]

A relevant comment https://github.com/celestiaorg/nmt/pull/209#issuecomment-1609561626

[vgonkivs]

It would be really nice if we will rename a json representation too, bc in celestia-node we've already had a request to make pb and json definitions consistent

[staheri14]

I am unassigning myself from this issue due to other ongoing high-priority big block issues.