Add XChaCha20 Cipherkey

MERGE REQUEST

Overview

Adds a XChaCha20 based implementation of crypto.CipherKey to be used in linkfilefanoutfetch.go / skyfilefanoutfetch.go

Every call to an encryption/decryption method resets the internal blockcounter. The blockIndex parameter in DecryptBytesInPlace can be used to skip ahead in the key stream, or the SetCounter method on the embedded chacha.Cipher type can be sued.

Example for Visual changes (ie Screenshot)

Issues Closed

First part of : #3811 (closed)

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods, or updating methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to work flow
• Issue added to Sia-UI repo for new supporting features
• [] Changelog updated