core: add builder registration duty type

[ciaranmcveigh5]

Jim has merged in the builder registration PR attestantio/go-eth2-client#19 so we are unblocked on that flow now

• Add DutyBuilderRegistration
• Update tests to include the registration duty

category: feature
ticket: #849

[codecov]

Codecov Report

Merging #852 (cb318eb) into main (f801a6d) will decrease coverage by 0.03%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##             main     #852      +/-   ##
==========================================
- Coverage   55.25%   55.21%   -0.04%     
==========================================
  Files         112      112              
  Lines       11724    11730       +6     
==========================================
- Hits         6478     6477       -1     
- Misses       4309     4316       +7     
  Partials      937      937              

Impacted Files	Coverage Δ
core/types.go	36.52% <58.33%> (-1.10%)	⬇️
core/dutydb/memory.go	70.63% <0.00%> (-1.28%)	⬇️
app/app.go	56.60% <0.00%> (+0.24%)	⬆️

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[corverroos]

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