Add namespace storage for upgradeable contracts

[minghinmatthewlam]

Context and scope
Implement ERC7201 namespace storage to upgradeable contracts to prevent storage collision between logic contract versions.

blocked by #397

Discussion and alternatives
Alternative was using _gap variables, but namespace storage has advantages:

Is the new standard and used in OZv5
For non-upgradeable extensions no unnecessary _gap variables
Ideally we'll include upgrading OZ version to v5, and sol version to ^0.8.20 as well, but this would also be compatible with OZ v4 with our current setup.