algod: Add API Endpoint to fetch TxIDs from block

[algochoi]

Summary

Adds an algod endpoint /v2/blocks/{round}/txids to return top level TxIDs in a given block.

#5461

Test Plan

Added a unit test in handlers_test.go, added a small test in an existing e2e test and locally tested endpoint to confirm that only top-level txids were reported.

{"blockTxids":["PFKS67KJXVXD3SMVP65F4NTNKULDBQJBBP3SCLHYFSTXDLQGX3BA"]}

[codecov]

Codecov Report

Merging #5611 (aad531d) into master (4dc3f92) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #5611   +/-   ##
=======================================
  Coverage   54.95%   54.96%           
=======================================
  Files         463      463           
  Lines       64553    64570   +17     
=======================================
+ Hits        35478    35491   +13     
- Misses      26698    26707    +9     
+ Partials     2377     2372    -5     

Files Changed	Coverage Δ
daemon/algod/api/server/v2/handlers.go	0.80% <0.00%> (-0.02%)	⬇️

... and 14 files with indirect coverage changes

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[tzaffi]

Looking clean and correct.

I left a couple of nits. Also consider adding a call to the endpoint in an already existing E2E test. But this might entail adding a local client (not too bad).

[winder]

LGTM

[tzaffi]

LGTM

[winder]

Declining for Pavel's feedback