use correct decimal or hex encoding/decoding for REST GetSpec

[tersec]

#5362

https://github.com/ethereum/beacon-APIs/blob/v2.4.2/apis/config/spec.yaml describes that

                  Values are returned with following format:
                    - any value starting with 0x in the spec is returned as a hex string
                    - numeric values are returned as a quoted integer

[github-actions]

Unit Test Results

         9 files  ±0    1 077 suites  ±0   37m 16s ⏱️ +18s
  3 725 tests ±0    3 446 ✔️ ±0  279 💤 ±0  0 ❌ ±0 
15 880 runs  ±0  15 575 ✔️ ±0  305 💤 ±0  0 ❌ ±0 

Results for commit ba13efc. ± Comparison against base commit f2d3859.

[tersec]

See also #5370, a slightly different take on the same idea