bump nim-json-serialization to `8a4ed98bbd0a9479df15af2fa31da38a586ea6d5`

[etan-status]

• Update nimbus-eth2 reference
• Add compile time switch to alter encoder enum representation

[github-actions[bot]]

Unit Test Results

         9 files  ±0    1 337 suites  ±0   42m 46s :stopwatch: - 1m 5s   5 066 tests ±0    4 718 :heavy_check_mark: ±0  348 :zzz: ±0  0 :x: ±0  20 997 runs  ±0  20 593 :heavy_check_mark: ±0  404 :zzz: ±0  0 :x: ±0 

Results for commit 73da5dd6. ± Comparison against base commit 73be2af0.