opentelemetry-exporter-otlp-common: add shared package for common OTLP utilities (#5252)

* opentelemetry-exporter-otlp-common: add shared package for common OTLP utilities

* add changelog fragment

* add typechecking

* update tox environment names

* fix missing content-encoding header

* fix docs build failures

* add shutdown() method and remove close() method and remove shutdown_event from client ctor

* update retry-after parsing