Add Mantle [don't merge] [duplicates fork]

[arwer13]

This PR duplicates fork changes provided by https://github.com/mantlenetworkio/lido-l2/commit/75e023ad54fc93de7f44d5c80339a3d59c4c7d31 to add more redundancy in keeping track of the code deployed on-chain.

It is not planned for merging (at least yet) due to the merge conflicts.