Add batch signing to Massa Wallet

[thomas-senechal]

Context
ATM, Massa Wallet can only sign a single operation at a time. For tools like DeWeb, it can be really annoying as it forces users to sign each chunk one by one, and a 1MB website is roughly 16 chunks, so 16 operations. Add one for the deployment. Really annoying.

Even if #102 aims to add the possibility to sign automatically using a SecretKey, as we plan to have a MS Plugin and/or a website to publish websites, we will still need the possibility to sign multiple operations at once.

How To

TDB

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