Forbid blocks over 2MiB in the Block API

[Stebalien]

Currently, we accept arbitrarily large blocks in the block API. We won't transfer them over the network so this isn't a big security concern, but it is a concern for users as they'll be able to add blocks that they can't transfer.

We should just forbid them in the block API.

[lidel]

• Addressed in v0.40.0 via fix: raise block size limits from 1MiB to 2MiB boxo#1101.
  • Block size limits are now enforced at 2MiB in the Block API (ipfs block put, ipfs dag put, ipfs dag import), matching the bitswap spec (https://specs.ipfs.tech/bitswap-protocol/#block-sizes).
  • Blocks over 2MiB require --allow-big-block. See the "Block size limit raised to 2MiB" section in the v0.40.0 changelog.