Implement off-chain user journey of incremental commits

[ch1bo]

Why

See #199

What

Implement the API endpoint and off-chain protocol changes, such that users can add funds to the open Hydra Head.

• Write tutorial on how to deposit and recover deposits using the hydra-node

[ch1bo]

When discussing incremental commits in #1484, we realized that any incremental commit "blueprint" transaction (the $tx_\alpha$) needs to be validated by the hydra-node when constructing the incrementTx (at latest; ideally even before requesting inclusion with a ReqInc). This can be seen as a form of input validation, while the protocol is secure by the fact that the L1 validates any value (requested to move) when applying the resulting incrementTx.

[ch1bo]

We might need to update the What here as #1571 led to some updates in #199.