Add Sepolia / Base Sepolia support for Base cross-chain (#742)

The Goerli testnet currently used by Threshold/Keep for development
purposes was planned to become deprecated with the end of year 2023.
Although it's already 2024, the testnet is still live, but it will loose
validators over the time and may become unusable at some point. The
testnet that was created to replace it is called
[Holešky](https://github.com/eth-clients/holesky), however it will take
some time until it gets integrated with by some of the projects we rely
on. As a solution, we decided to switch first to another testnet that is
currently live - Sepolia. This testnet's EOL is planned for 2026, which
gives us plenty of time to move to Holešky before Sepolia gets
deprecated.

The Goerli -> Sepolia migration means that also L2 testnet based on
Goerli (Base Goerli) needs to be migrated to Sepolia-based chain (Base
Sepolia). In this PR we're adjusting the config used to deploy testnet
contracts in the `@keep-network/tbtc-v2-arbitrum` project. After the
update, both deploy to `baseGoerli` and `baseSepolia` networks will be
supported (we may remove `baseGoerli` support once we're sure the
`baseSepolia` deployment works as expected).

TODO
- [x] Add `external/baseSepolia/BaseWormholeTBTC.json` - in order to get
the address we need to perform attestation of the Ethereum Sepolia TBTC
token on Base Sepolia (but atm Base Sepolia is not available on the list
of target networks on
https://wormhole-foundation.github.io/example-token-bridge-ui/#/register)

Ref: threshold-network/solidity-contracts#150

cross-chain/base/.gitignore

@@ -10,8 +10,10 @@
 !/deployments/mainnet/
 !/deployments/base/
 !/deployments/baseGoerli/
+!/deployments/baseSepolia/
 
 # OZ
 /.openzeppelin/unknown-*.json
 !/.openzeppelin/unknown-84531.json
 !/.openzeppelin/unknown-8453.json
+!/.openzeppelin/unknown-84532.json