Rewrite exit scanner to produce exit lists according to new format

The current exit scanner is purpose written software using Haskell. It is non-trivial to update this program to run on a modern Debian system and would be easier in the longer term to simply replace.

This task requires the new exit list specification to be completed in #29624 (moved).

Expected new features:

• Written in Python
• Support for IPv6
• Support for new exit list format

Features we would like:

• Cryptographically signed measurement results
• Archival signatures