Implement header interface for block and add block exchange service

[Manav-Aggarwal]

Overview

Resolves #1005. resolves #1021

Checklist

• New and updated code has appropriate documentation
• New and updated code has new and/or updated testing
• Required CI checks are passing
• Visual proof for any user facing features like CLI or documentation updates
• Linked issues closed with keywords

[codecov]

Codecov Report

Patch coverage: 51.17% and project coverage change: -0.23 ⚠️

Comparison is base (e4f8bd4) 54.95% compared to head (2f1c1c7) 54.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1014      +/-   ##
==========================================
- Coverage   54.95%   54.73%   -0.23%     
==========================================
  Files          60       63       +3     
  Lines        6423     6628     +205     
==========================================
+ Hits         3530     3628      +98     
- Misses       2537     2621      +84     
- Partials      356      379      +23     

Impacted Files	Coverage Δ
node/header_exchange.go	53.07% <ø> (ø)
types/block.go	0.00% <0.00%> (ø)
block/manager.go	9.16% <25.00%> (+0.13%)	⬆️
node/block_exchange.go	51.40% <51.40%> (ø)
node/full.go	68.96% <72.72%> (+0.39%)	⬆️
node/test_helpers.go	80.64% <93.75%> (+1.01%)	⬆️
node/exchange.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[MSevey]

Looks good. Would look even better with tests 😉

[Manav-Aggarwal]

Extracted the changes made to refactor header_exchange.go into #1028 since they make a test, LazyAggregator, in full_node_integration_test.go fail. Also, refactored tests to reuse the TestHeaderExchange tests in a new test called TestBlockExchange for testing Block Exchange Service.