header: Exchange interface implements Getter

[renaynay]

This PR changes Exchange interface to just implement Getter as they are performing the same functions just with different names.

[codecov-commenter]

Codecov Report

Merging #842 (02287af) into main (643749f) will decrease coverage by 0.10%.
The diff coverage is 88.23%.

@@            Coverage Diff             @@
##             main     #842      +/-   ##
==========================================
- Coverage   54.00%   53.90%   -0.11%     
==========================================
  Files         122      122              
  Lines        6979     6979              
==========================================
- Hits         3769     3762       -7     
- Misses       2823     2828       +5     
- Partials      387      389       +2     

Impacted Files	Coverage Δ
header/local/exchange.go	68.42% <75.00%> (ø)
header/core/exchange.go	50.00% <80.00%> (ø)
header/p2p/exchange.go	62.22% <100.00%> (ø)
header/store/init.go	100.00% <100.00%> (ø)
header/sync/sync.go	65.34% <100.00%> (ø)
fraud/bad_encoding.go	40.59% <0.00%> (-2.98%)	⬇️
das/daser.go	73.68% <0.00%> (-2.64%)	⬇️

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[Wondertan]

❤️ this change, even though it has a low priority.