fix(trading-sdk): fix order amounts calculations

[shoom3301]

Fixes first issue from the doc.

This change fixes both sell and buy orders.
It also fixes "Private Key example not working" issue.

Basically, I reverted this commit: 718b212

Unfortunately, I don't remember why I did this :(
But obviuosly this logic is wrong, because it adds fees + slippage twice:

• once in swapParamsToLimitOrderParams
• and another one in getOrderToSign (getQuoteAmountsAndCosts call)

I compared the result with CoW Swap and it matches 100%.
How to test that:

1. Copy quoteResponseMock from unit test
2. Go to apps/cowswap-frontend/src/api/cowProtocol/api.ts in CoW Swap
3. Return the value of copied mock from getQuote() function
4. Open CoW Swap and setup a trade correspondingly to the mock
5. Init order signing
6. Compare sell and but amounts in the wallet with values in unit tests

[coveralls]

coverage: 78.463% (+0.09%) from 78.378%
when pulling ad70522 on fix/trading-sdk-amounts
into c379396 on main.

[anxolin]

Cool, I will test in the script after publishing to NPM. If you want earlier validation, I could debug it locally.

Let me know

[anxolin]

Is this soimething we want to mock always or just in this file?

[shoom3301]

The mock is specific for sell/buy orders, so I don't think we can use it by default

[shoom3301]

Cool, I will test in the script after publishing to NPM

@anxolin appreciate it 🙏