Fix and refactor webhook validation

[unspok3n]

Thanks for this awesome lib! Currently, the webhook validation seems to be a little bit broken because of the wrong types, missing fields in the Webhook struct and a typo in the (c *Cryptomus) VerifySign function (passing unmodified reqBody to the sign func)

This PR:

• Makes nullable fields in the webhook struct pointers
• Adds missing undocumented webhook fields (payer_amount, payer_amount_exchange_rate, transfer_id)
• Moves verification logic from (c *Cryptomus) VerifySign to (w *Webhook) verify function
• Makes signRequest a regular function (it didn't use any values from the receiver), so it can be called from (w *Webhook) verify
• Escapes forward slash after marshalling modified webhook (txid contains a slash that has to be escaped https://doc.cryptomus.com/business/payments/webhook)
• Adds webhook verification example to the README