Plasma bridge update

[simonDos]

Plasma Bridge Modifications

The plasma bridge will be modified to bridge POL for users trying to bridge MATIC after the upgrade.

The following changes will be made:

Changes	Implementation
1. The entire MATIC balance held by the bridge will be converted to POL. This also has the added benefit that there is a sufficient amount of MATIC for unmigration for the StakeManager rewards.	A publicly callable function to trigger the conversion has been written for this.
2. When depositing MATIC, automatically convert it to POL on L1 and bridge MATIC	A convenience functionality has been added in _createDepositBlock, which automatically converts MATIC to POL when a user tries to bridge MATIC.
3. When depositing POL, bridge MATIC as well	Is caught in the _createDepositBlock as well, changing the token address to MATIC before syncState
4. When bridging MATIC back to L1 (withdrawing), always pay out POL	Additional functionality in transferAssets to pay out POL instead of MATIC

Internally, the MATIC address will still be used for all POL bridging operations, as the behaviour of the native token on the PoS side will not be modified and thus can keep operating as usual.

💡 Action items (for deployment):

1. call updateContractMap on the Registry to add
   “matic”, “pol” and “polygonMigration”
2. call mapToken on the Registry to map POL to native MATIC
3. deploy new DepositManager version and update proxy
4. call migrateMatic on the new DepositManager, migrating all MATIC