Update aws-sigv4 to `>= 0.57`

[Xtansia]

Description

Update aws-sigv4 to >= 0.57

• Expose signing time source as configurable for testability.
• Explicitly add global default headers upfront rather than via reqwest::Client so that they are taken into account for signing.
• Add test cases for sigv4 based off those in opensearch-net.

Issues Resolved

Fixes #200

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[codecov[bot]]

Codecov Report

Merging #201 (bf4f0c5) into main (1041309) will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #201   +/-   ##
=======================================
  Coverage   73.83%   73.83%           
=======================================
  Files         403      403           
  Lines       63772    63777    +5     
=======================================
+ Hits        47088    47092    +4     
- Misses      16684    16685    +1     

Flag	Coverage Δ
integration	73.83% <ø> (+<0.01%)	:arrow_up:

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see 1 file with indirect coverage changes