feat: initial utils for reading/writing e2s files

[njgheorghita]

What was wrong?

In portal-bridge we'll soon need the ability to read/write era1 files. These are the initial set of utils for reading/writing e2s files, which are the standard format that era1 files are based on

How was it fixed?

• Implemented basic encode/decode functionality for e2s files
• Added standard test vectors

To-Do

• Clean up commit history and use conventional commits.

[KolbyML]

nothing stands out as wrong. I want to get this in so we (nick/ogy) can start iterating on it