add Electra SSZ object test fixture

[tersec]

• AggregateAndProof/SignedAggregateAndProof are not actually there, just stubbed out; probably another one of the base-to-phase0-then-explicitly-specify-module maneuvers coming;
• CommitteeValidatorsBits | ElectraCommitteeValidatorsBits could be combined into a single typeclass, probably, though Nim does have bugs here. They've not lately been breaking-at-runtime, i.e. if it compiles, this at least works across modules;
• various internal data structures will need to be rethought to the extent they'll need queues of both phase0 and electra versions of Attestations/AttesterSlashings, but at least no pre-Electra regressions this way, by pinning them to the phase0 versions;
• ElectraAttesterSlashing and similar will become electra.Foo rather than ElectraFoo

[github-actions]

Unit Test Results

         9 files  ±    0    1 121 suites  +6   34m 7s ⏱️ + 3m 1s
  4 252 tests +    7    3 905 ✔️ +    7  347 💤 ±0  0 ❌ ±0 
17 259 runs  +330  16 861 ✔️ +330  398 💤 ±0  0 ❌ ±0 

Results for commit b169ebd. ± Comparison against base commit d139c92.