chore: use Etherscan API V2 for contract verification

[anxolin]

What

Adds an [etherscan] section to foundry.toml so forge verify-contract / forge script --verify use the Etherscan API V2 endpoint for all supported chains (mainnet, gnosis, sepolia, arbitrum, bnb, linea).

Why

The legacy per-explorer V1 APIs (e.g. api.gnosisscan.io) have been shut down. Verifying against them now fails with:

You are using a deprecated V1 endpoint, switch to Etherscan API V2

V2 uses a single host (api.etherscan.io/v2/api) with the chain id passed as a query parameter, and a single etherscan.io API key works across every chain. This config points each chain at the V2 endpoint so verification works out of the box.

Test plan

1. Set a V2 key from https://etherscan.io/myapikey:

   export ETHERSCAN_API_KEY=<your etherscan.io key>

2. Verify any already-deployed contract (example: Gnosis), now resolving via the config:

   forge verify-contract <ADDRESS> src/types/twap/TWAP.sol:TWAP \
     --chain gnosis \
     --constructor-args <ABI_ENCODED_ARGS> \
     --watch

   It should resolve the verifier URL to https://api.etherscan.io/v2/api?chainid=100 and succeed instead of returning the V1 deprecation error.
3. Confirm the config parses:

   forge config | grep -A1 '\[etherscan'