Add required header for SigV4 signing

[harshavamsi]

Description

Fixes missing header for Amazon OpenSearch Serverless

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

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[codecov-commenter]

Codecov Report

Merging #119 (0125698) into main (fa650c2) will increase coverage by 0.01%. The diff coverage is 100.00%.

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@@             Coverage Diff              @@
##               main     #119      +/-   ##
============================================
+ Coverage     23.89%   23.90%   +0.01%     
  Complexity     1936     1936              
============================================
  Files           245      245              
  Lines          6856     6857       +1     
============================================
+ Hits           1638     1639       +1     
  Misses         5218     5218              

Impacted Files	Coverage Δ
src/OpenSearch/Handlers/SigV4Handler.php	84.90% <100.00%> (+0.29%)	:arrow_up:

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[harshavamsi]

@dblock we've already added tests/changelog/user_guide in a prior PR. This just fixes an issue as part of that.

[dblock]

@dblock we've already added tests/changelog/user_guide in a prior PR. This just fixes an issue as part of that.

I don't think I agree ;)

1. This code adds a new x-amz-content-sha256, there are no tests that require that this header is present and is correct. If someone removes this header inadvertently in a refactor or because they don't know that it's required by serverless, no tests will fail.
2. ~There's no documentation that mentions that this library supports Serverless. So users don't know if it works.~
3. ~There's no entry in CHANGELOG that says that serverless is now supported. Which is a change.~

I'll stay corrected on 2) and 3) which were added in #117, but you want to add this PR to the list still in CHANGELOG.

[harshavamsi]

@dblock @shyim this should be good now.

[shouze]

@harshavamsi thank you, using your fork make us able to user Amazon OpensSarch Serverless :tada: