eth2util: support legacy signed randao marshalling

[corverroos]

Workaround to marshal SignedRandao using legacy [96]byte signature marshalling to remain compatible with v0.16.

This was discovered by go-eth2-client that added proper json marshalling using 0xhex strings for eth2p0.BLSSignature.

This starts the following migration:

• v0.16 marshals legacy and unmarshals legacy only.
• v0.17 marshals legacy, but supports unmarshalling both new and legacy.
• v0.18 will marshal new, but will support unmarshalling both new and legacy
• v0.19 will marshal new and will unmarshals new only

category: bug
ticket: #2333

[codecov]

Codecov Report

Patch coverage: 27.77% and project coverage change: +0.21 🎉

Comparison is base (7374a44) 53.49% compared to head (42d13cf) 53.71%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2353      +/-   ##
==========================================
+ Coverage   53.49%   53.71%   +0.21%     
==========================================
  Files         192      192              
  Lines       25847    25862      +15     
==========================================
+ Hits        13827    13891      +64     
+ Misses      10308    10249      -59     
- Partials     1712     1722      +10     

Impacted Files	Coverage Δ
eth2util/types.go	43.47% <27.77%> (+11.22%)	⬆️

... and 6 files with indirect coverage changes

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