Implement AppData Spec v0.5.0

[nlordell]

This PR implements the new AppData spec described in https://github.com/cowprotocol/cowswap/issues/1542. Specifically, this adds a new class field to the Metadata type for including information on the order class (either market, limit or liquidity order).

I also added a few unrelated changes (can split into a separate PR if necessary, but they were in related code). Namely:

• Remove environment from Metadata, according to https://github.com/cowprotocol/app-data, it was never there in the first place
• Stricter version parsing

Test Plan

Added new unit tests to test serialization. Other than that, no real logic changes to test.

[bh2smith]

Cool idea @anxolin - I am all for it if we can make it work, although that would be a much bigger change than we are seeing here already. Could be a fun task for later though (and this part of the project is going to be archived soon).

Once this is merged, we should tag a new version and prepare a release to production. Luckily, I don't believe this change requires a full version bump. Just let me know when we want to proceed.

[bh2smith]

Production release merge request is here.