Add UnsafeUpgrades library variations, add LegacyUpgrades.sol

[ericglau]

Fixes #2
Fixes #44
Fixes #34

Overview of changes:

• Handle UPGRADE_INTERFACE_VERSION in upgrade functions.
• Add LegacyUpgrades.sol to support upgrading existing deployments using OpenZeppelin Contracts v4.
• Adds UnsafeUpgrades library variations in both Upgrades.sol and LegacyUpgrades.sol to work with forge coverage.
  Note that UnsafeUpgrades does not perform any upgrade safety validations, so is not recommended for use with Forge scripts.

Summary of how this was implemented:

• Upgrades.sol imports proxies from OpenZeppelin Contracts v5 and its functions help to deploy those. Users who import this file must have OpenZeppelin Contracts v5, otherwise compilation would fail.
• LegacyUpgrades.sol does not import proxy contracts, but only supports upgrading existing implementations or proxies which use OpenZeppelin Contracts v4. Users can import this file if they are using OpenZeppelin Contracts v4.
• Upgrades and UnsafeUpgrades libraries exist in both Upgrades.sol and LegacyUpgrades.sol.
• Testing for this project itself is separated into Foundry profiles (see foundry.toml), which allows us to use profile-specific remappings to test with different versions of OpenZeppelin Contracts for the proxies and/or for implementations, along with profile-specific test cases.

[Amxx]

LGTM