perf(lambda-at-edge): do not decompress content from fetch in externa…

[dphang]

…l rewrite.

• Since we aren't doing anything with the response body any, there's no need to decompress it. Send it back to CF response as buffer and do not decompress it.
• If already compressed, it will reuse the existing compression (e.g brotli or gzip) whereas before it would decompress + then rely on CF to re-compress it
• If not already compressed, it will be same behavior and CF will auto-compress specific content types (e.g json, html, txt)

This should help with performance and I believe should help size issues like: #870 (as long as content can be compressed)

[codecov]

Codecov Report

Merging #871 (b682adf) into master (42c9276) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #871      +/-   ##
==========================================
- Coverage   80.50%   80.49%   -0.01%     
==========================================
  Files          63       63              
  Lines        2257     2256       -1     
  Branches      513      513              
==========================================
- Hits         1817     1816       -1     
  Misses        378      378              
  Partials       62       62              

Impacted Files	Coverage Δ
...ckages/libs/lambda-at-edge/src/routing/rewriter.ts	100.00% <100.00%> (ø)

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