New module: base64-dns

[chrisforce1]

As per https://github.com/krmaxwell/dns-exfiltration we should synthesize Base64 encoding and exfiltration of data to hostnames under base64.alphasoc.xyz, as below:

1. Generate a long random binary value from /dev/random or similar
2. Chop the value into pieces and encode each with Base64
3. Ship each piece out over DNS (e.g. AAAAAAAAAAAxMjM0NTY3OA==.base64.alphasoc.xyz)

Module description for the table in the documentation as below.

Module	Description
base64-dns	Exfiltrates Base64-encoded data over DNS to *.base64.alphasoc.xyz

We should probably rename sandbox.alphasoc.xyz to tunnel.alphasoc.xyz too. Thoughts?

[chrisforce1]

This is a lower priority as it is blocked by https://github.com/alphasoc/riswiz/issues/321.