API endpoint for UDC contract to deposit and withdraw RDN tokens

[Dominik1999]

We need an API endpoint to be able to deposit RDN tokens (or services tokens on the testnet) into the UDC.

This is related to #5407

[manuelwedler]

I'm taking this over now to provide a better UX. After a discussion with @karlb, we decided to have 3 API calls: deposit, plan withdraw and withdraw.

For this it makes sense to have a new endpoint. I propose for it to be /api/<version>/user_deposit.

Methods:

• POST /api/<version>/user_deposit
{total_deposit: <amount>}
• POST /api/<version>/user_deposit
{planned_withdraw_amount: <amount>}
• POST /api/<version>/user_deposit
{withdraw_amount: <amount>}

The first two are idempotent. The parameters are exactly what the respective contract functions expect. The requirements for doing the transactions (e.g. there must be a planned withdraw before withdrawing) should get checked in the API before sending a transaction to avoid it to be failing.

As we do with the other API calls which trigger an on-chain transaction we should wait for the transaction to be mined before returning the response. The call for planning a withdraw can then respond with the number of blocks for it to be ready.