registry: use more efficient json.Unmarshal on manifest

Created by: keegancsmith

I took a random trace of sourcegraph.com, and this was the largest use of CPU. There are more optimizations we can make here, but the simple change is to just use a more efficient unmarshaller. I validated that all data in postgres is normal json (instead of jsonc). Additionally the column type is jsonb, so it should always be conformant json.

I also checked how often we need to blend in the URL (the purpose of this function). The answer is most of the time.

Part of https://github.com/sourcegraph/sourcegraph/issues/7544

Created by: codecov[bot]

Codecov Report

Merging #8378 into master will not change coverage. The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #8378   +/-   ##
=======================================
  Coverage   40.66%   40.66%           
=======================================
  Files        1291     1291           
  Lines       69035    69035           
  Branches     6354     6354           
=======================================
  Hits        28070    28070           
  Misses      38366    38366           
  Partials     2599     2599

Impacted Files	Coverage Δ
...d/frontend/internal/registry/extension_manifest.go	55.88% <0%> (ø)