Currently we use subnets as a unit of custody in the PeerDAS core
protocol because it doesn't make sense to partially custody only some
columns in the subnets and waste the bandwidth to download the columns
the node doesn't custody.
Since subnets correspond to GossipSub topics which are in a layer lower
than the core protocol, using subnets as a unit of custody makes the
core layer and the network layer too coupled to each other and leave no
room for the network layer flexibility.
This commit introduces "custody groups" which are used a unit of custody
instead of subnets.
The immediate benefit of the decoupling is that we can immediately
increase the number of subnets without affecting the expected number of
peers to cover all columns and affecting the network stability and
without touching the core protocol.
The reason we want to increase the number of subnets to match the number
of columns is that the columns will be propagated through the network
faster when they have their own subnets. Just like EIP-4844, each
blob has its own subnet because, if all the blobs are in a single subnet,
the blobs will be propagated more slowly.
Since we keep the number of custody groups the same as the previous
number of subnets (32), the expected number of peers you need to cover
all the columns is not changed. In fact, you need only NUMBER_OF_COLUMNS
and NUMBER_OF_CUSTODY_GROUPS to analyze the expected number, which
makes the core protocol completely decoupled from the network layer.
