feat(test-specs): add container definitions for ssz-engine api

[RazorClient]

SSZ container definitions and helpers for the REST+SSZ Engine API refactor execution-apis PR.
This is foundational plumbing for the upcoming REST+SSZ blockchain-test support;
it does not add any fillable test cases yet.

🔗 Related Issues or PRs

Tracks execution-apis PR

✅ Checklist

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:
  uvx tox -e static
• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.
• Tests: For PRs implementing a missed test case, update the post-mortem document to add an entry the list.
• Ported Tests: All converted JSON/YML tests from ethereum/tests or tests/static have been assigned @ported_from marker.

Cute Animal Picture

[RazorClient]

this file does the work of https://github.com/ethereum/consensus-specs/blob/master/tests/core/pyspec/eth_consensus_specs/debug/random_value.py and is needed to have the static ssz tests with https://github.com/ethereum/consensus-specs/blob/master/tests/core/pyspec/eth_consensus_specs/test/phase0/ssz_static/test_ssz_static.py in the next prs

[jtraglia]

Should the BuiltPayload containers really exist? Will clients have this?

[jtraglia]

Here, I would actually suggest doing from .constants import *. Maybe EELS will disagree. I expect everything from that file will always be imported/used.

[RazorClient]

star exporting would work if we use all the exports. If not, then ruff marks every use as F405 (it can't trace names through import *)

[jtraglia]

Shouldn't these be SSZ types?

[RazorClient]

envelope_bytes is intentionally a plain-values builder: callers pass ordinary bytes/int, and it wraps them into the SSZ containers internally, so plain parameter types are the arguments

[jtraglia]

Docstrings like these are not useful. I would suggest removing docstrings from all containers in this file.

[RazorClient]

The repo has pydocstyle D101 globally enabled, so every public class must have a docstring.