feat: add opentelemetry-exporter-http-transport package (#5194)

* feat: add opentelemetry-exporter-http-transport package

* add remaining unit tests

* improve unit tests

* update README.rst

* update CHANGELOG.md

* update github workflows

* update eachdist.ini

* update test-requirements.txt

* remove pypy from tests

* run generate workflows

* add test requirements latest/oldest

* update tests to use mocket

* fix lint errors

* remove OTLPHTTPClient and update ABCs

* refactor unit tests

* update package version

* remove future annotations imports

* small tweak to headers behavior for transport implementations

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Co-authored-by: Leighton Chen <lechen@microsoft.com>