Roadmap for pruning historical data blobs

The data storage model for data availability networks is that the data availability network is only expected to store data blobs for a certain period of time ("**storage window**"), and [purge data blobs older than the storage window](https://notes.ethereum.org/@vbuterin/proto_danksharding_faq#If-data-is-deleted-after-30-days-how-would-users-access-older-blobs). The storage window should probably be greater or equal to the unbonding period, at minimum. However currently in celestia-node, all full nodes store all historical data, and all light nodes sample all historical block headers.

We propose a roadmap in which we can move to pruning historical data blobs.

**Milestone 1: light node sampling only within the storage window by default.**
By default, make it so that light nodes only sample (but still download) historical headers within the storage window. If they have been offline more than the unbonding period, they need a trusted hash anyway.

**Milestone 2: implement full node pruning of blobs outside of the storage window.**
Add the option for full nodes to prune historical blobs outside the storage window. Provide a way for nodes to discover non-pruned full nodes as well as pruned full nodes, so that nodes can still get historical blobs if it's available.

**Milestone 3: implement namespaced partial storage full nodes.**
Add the option for full nodes to follow and store the data for a set of specified namespaces, and allow themselves to advertise themselves for that namespace. This will allow rollup full nodes to be able to take responsibility for storage of historical data blobs.

**Milestone 4: implement [equally-sharded partial storage nodes](https://github.com/celestiaorg/celestia-node/issues/1133) within the storage window.**
Like milestone 3, but instead of storing by namespace, store entire blocks based on what part of the chain they are. In this milestone, we are only focused on blobs within the storage window. This is so that we can increase the block size without increasing the requirements for people to run nodes that can serve light node sampling requests.

**(stretch) Milestone 5: implement equally-sharded partial storage nodes outside of the storage window.**
Like milestone 4, but for old blobs outside of the storage window. This will provide functionality that is outside the core guarantees of the data availability layer, by providing a means to download historical data blobs outside of the promised storage window. However, this may still be useful as a protocol for pay-to-play historical blob storage providers, e.g. you can only connect to such nodes if you pay them. This is also basically the same as non-pruned nodes in Milestone 2, but sharded.