PoV size test

[xlc]

assert the PoV size for empty block is < X assert the PoV size for block with 100 transfer is < Y assert the PoV size for block with Z transfer is < 5mb (i.e. we can have Z transfers in a block)

We need to build a block, generate storage proof, compares it, and assert the size.

https://github.com/paritytech/cumulus/commit/794bc23de46e252a5a5551e1fa5d13a73ebada42#diff-0ca4be53dcb6c6cf8044a1b5357ab435d7c81c778a19f1bf31789d358f6f4f16R443 https://github.com/paritytech/cumulus/blob/a7b44a9effba21d6ffea2fdcb3a7f46cf4cfc2bd/client/collator/src/lib.rs#L442 https://github.com/paritytech/polkadot/blob/c1940f13aeebf1e46be6f65fe14c0cbd65af8c77/node/collation-generation/src/lib.rs#L318