Require MaxEncodedLen per default

[ggwpez]

Many pallets are currently missing MaxEncodedLen implementations. All future pallets should have it.
To make it easier not to miss it, we could either:

• Add an optional bool argument to generate_storage_info which default to true
• Remove generate_storage_info and add a without_storage_info attribute

What do you favor?

I personally like the second approach more, since we could then in the future remove both after all pallets have been migrated.

Related paritytech/polkadot-sdk#323

[shawntabrizi]

I strongly prefer the second option. By default, we should make sure all pallets follow this storage definition requirement.