Support gs_multi_send in the safe-apps-provider

[mmv08]

We are migrating the wallet-connect safe app to the safe-apps-provider:
safe-global/safe-react-apps#537

The only thing the safe-apps-provider lacks is the support for the OG gs_multi_send method for batching transactions. Supporting this method has some complexity because under the hood, the safe-apps-provider stores a fake transaction that includes the recipient address, data, etc. Since the transactions passed to the gs_multi_send method will be encoded and sent to the multisend contract, we need somehow to get the encoded data and the address of the multisend contract.

I see two options to achieve this:

1. Replicate the encoding/address retrieval logic from our interfaces in the safe-apps-provider
2. Change the txs.send return type to return the whole safe transaction object

I lean towards 2)