Make zkp_body_root available to light clients

[paberr]

Light clients in the browser are slowed down by the decision to move the Validators object into the header as they now need to compute the pk-tree root instead of taking it out of the header.
#3121 addressed this partially by improving the caching of the block hash.
However, it would be even better if we could make the zkp_body_root available to light clients.

[paberr]

Light nodes are using the validators during macro sync to verify subsequent blocks. So, we should actually check this hash.