add raw json requests docs

[samuelorji]

Description

Add documentation on how to make raw json / http requests using the client

Issues Resolved

https://github.com/opensearch-project/opensearch-rs/issues/193

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[codecov[bot]]

Codecov Report

Merging #196 (8f2fb66) into main (13d0d4b) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #196   +/-   ##
=======================================
  Coverage   73.82%   73.82%           
=======================================
  Files         402      402           
  Lines       63738    63738           
=======================================
  Hits        47055    47055           
  Misses      16683    16683           

Flag	Coverage Δ
integration	73.82% <ø> (ø)

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[samuelorji]

Looks like we call the samples directory samples in other clients, not examples.

I'm not a Rust expert, but I think in Rust, the convention is to put samples in the examples folder