Add Proxies from OpenZeppelin SDK

[frangio]

As mentioned in #2207, we're moving the proxies from OpenZeppelin SDK into the Contracts project so they can 1) be more readily accessible to users, and 2) benefit from the community participation in maintenance and security assessment.

This PR is a simple port of the contracts and their tests from the SDK repo, the main difference being the removal of the "base" and "initializable" distintion (see here) for simplicity.

Some potential renames:

• UpgradeabilityProxy → EIP1967Proxy (or ERC1967Proxy?)
• AdminUpgradeabilityProxy → AdminTransparentProxy

[spalladino]

LGTM @frangio! My thoughts on naming:

Proxy
I'd rename to DelegateProxy, as opposed to a ForwardProxy

UpgradeabilityProxy
I'd consider changing to UpgradeableProxy or UpgradeProxy. EIP1967 is tempting, but it's only part of the implementation. For instance, UUPS is 1967 compatible, but uses a different approach where upgradeTo is not part of the proxy.

AdminUpgradeabilityProxy
I really like the idea of including Transparent on this one. AdminTransparentProxy sounds fine, and I'd also consider TransparentUpgradeProxy (or TransparentUpgradeableProxy).

Last: have you considered moving Initializable out of /proxy? Looks odd in there. But I'm not sure where else to place it.

[frangio]

@spalladino Initializable could go in utils as well. But we could also leave it in proxy, since that's really what it's designed for.