Fix oapi-codegen installation.

[winder]

Summary

The REST API generate command is failing to install oapi-codegen because it needs an older version of kin-openapi than we define in go.sum. I'm attempting to install it with the buildtools script.

Also added REST server generation to the codegen test.

Test Plan

CI

[codecov-commenter]

Codecov Report

Merging #2668 (f17aa05) into master (d1425f0) will increase coverage by 0.03%.
The diff coverage is n/a.

❗ Current head f17aa05 differs from pull request most recent head e9eed05. Consider uploading reports for the commit e9eed05 to get more accurate results

@@            Coverage Diff             @@
##           master    #2668      +/-   ##
==========================================
+ Coverage   47.00%   47.04%   +0.03%     
==========================================
  Files         349      349              
  Lines       55814    55814              
==========================================
+ Hits        26238    26256      +18     
+ Misses      26623    26608      -15     
+ Partials     2953     2950       -3     

Impacted Files	Coverage Δ
ledger/acctupdates.go	61.88% <0.00%> (-0.42%)	⬇️
catchup/service.go	69.35% <0.00%> (ø)
network/wsNetwork.go	60.92% <0.00%> (ø)
data/transactions/verify/txn.go	48.45% <0.00%> (ø)
cmd/tealdbg/debugger.go	73.86% <0.00%> (+1.00%)	⬆️
ledger/blockqueue.go	82.18% <0.00%> (+1.14%)	⬆️
crypto/merkletrie/node.go	93.48% <0.00%> (+1.86%)	⬆️
crypto/merkletrie/trie.go	68.61% <0.00%> (+2.18%)	⬆️
network/wsPeer.go	75.48% <0.00%> (+3.34%)	⬆️

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[tsachiherman]

Looks good