fix: gateway/blocks-backend: GetBlock should not perform IPLD decoding

[gsergey418alt]

Fixes #673.

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.

Project coverage is 60.51%. Comparing base (24b6bc3) to head (b76f2d0).

Files with missing lines	Patch %	Lines
gateway/backend_blocks.go	78.57%	2 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main     #845      +/-   ##
==========================================
+ Coverage   60.37%   60.51%   +0.14%     
==========================================
  Files         243      244       +1     
  Lines       31119    31154      +35     
==========================================
+ Hits        18787    18853      +66     
+ Misses      10648    10623      -25     
+ Partials     1684     1678       -6     

Files with missing lines	Coverage Δ
gateway/backend_blocks.go	46.16% <78.57%> (+0.60%)	⬆️

... and 12 files with indirect coverage changes

[gammazero]

Tested in kubo CI also. Looks good.

@gsergey418alt Thank you for this.