fail to validate new dymint header when upgrade

[srene]

due to upgrade flow, header validation requires to pass validation for both previous and new drs, since the block produced with for the first block of the upgrade is created with the previous drs version. since headers are not backward compatible validation fails when upgrading to new drs and full-nodes need to validate previous drs (first after the upgrade) block, with new drs version.

[srene]

to solve this issue it would be required to add block versions, so depending of the version of the the block a specifc validation for the specific version will need to run. however, as a temporary solution, we will add a flag for full-nodes to skip a specific block height validation, to be used with the new drs version at the upgrade height. it does not imply security threat since the block was already validated before changing to the new version binary.