Redis: save ExecPayload in SSZ format (requires update of both builder-API and proposer-API ⚠️)

[metachris]

📝 Summary

Saving CapellaExecutionPaylaod SSZ-encoded in Redis should be a small performance improvement.

⚠️ Important: changes need to get rolled out to builder-API and proposer-API simultaneously ⚠️

This PR changes the Redis storage key for execution payloads, where the proposer API looks up the getPayload response

The recommended update sequence is as follows:

1. disable builder-API ingress
2. update proposer-API and builder-API
3. enable builder-API ingress

---

✅ I have run these commands

• make lint
• make test-race
• go mod tidy
• I have seen and agree to CONTRIBUTING.md

[codecov-commenter]

Codecov Report

Merging #448 (a219148) into redis-pipeline-2 (8c8dbeb) will decrease coverage by 0.07%.
The diff coverage is 29.62%.

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@@                 Coverage Diff                  @@
##           redis-pipeline-2     #448      +/-   ##
====================================================
- Coverage             29.74%   29.68%   -0.07%     
====================================================
  Files                    24       24              
  Lines                  4693     4706      +13     
====================================================
+ Hits                   1396     1397       +1     
- Misses                 3108     3119      +11     
- Partials                189      190       +1     

Flag	Coverage Δ
unittests	29.68% <29.62%> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datastore/datastore.go	0.00% <0.00%> (ø)
datastore/redis.go	58.01% <30.76%> (-1.37%)	⬇️