feat(spec-tools): support arbitrary headers in sync

[SamWilsn]

🗒️ Description

To interact with some RPC providers, we need to add additional headers to our requests.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.31%. Comparing base (85c6fff) to head (48865bd).
⚠️ Report is 3 commits behind head on forks/osaka.

Additional details and impacted files

@@             Coverage Diff              @@
##           forks/osaka    #1839   +/-   ##
============================================
  Coverage        87.31%   87.31%           
============================================
  Files              541      541           
  Lines            32832    32832           
  Branches          3015     3015           
============================================
  Hits             28668    28668           
  Misses            3557     3557           
  Partials           607      607           

Flag	Coverage Δ
unittests	87.31% <ø> (ø)

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