feat: multiple allowed cash tokens

[deluca-mike]

• DualGovernorDeployer is now deployed with an array of allowedCashTokens_, which are a permanent set for the system.
• This array is converted into a truthy hashmap in the DualGovernor storage in its constructor.
• The first cash token in the array of allowedCashTokens_ is the default for any new DualGovernor deployment (i.e. post-reset).
• Registrar no longer needs a cashToken in its constructor, as it is read when needed form the DualGovernorDeployer.
• setCashToken(address newCashToken_) is now a Zero type proposal.
• PowerToken has a method setNextCashToken that only its governor can call, which ensures the new cash token takes effect only in the next epoch and onwards, guaranteeing the continued and consistent expected operation of the in-progress auction.

[github-actions]

LCOV of commit 7da264d during Forge Coverage #566

Summary coverage rate:
  lines......: 67.1% (320 of 477 lines)
  functions..: 55.7% (123 of 221 functions)
  branches...: no data found

Files changed coverage rate:
                                         |Lines       |Functions  |Branches    
  Filename                               |Rate     Num|Rate    Num|Rate     Num
  =============================================================================
  src/DualGovernor.sol                   |66.2%    145|45.2%    73|    -      0
  src/DualGovernorDeployer.sol           |80.0%      5|75.0%     4|    -      0
  src/PowerToken.sol                     |83.6%     61|73.1%    26|    -      0
  src/Registrar.sol                      | 100%     18| 100%     7|    -      0

[toninorair]

looks good. Let's see if there is a way to not set next cash token next week.

The base branch was changed.