feat: add 'opentelemetry-exporter-otlp-json-common' package (#4996)

* feat: add 'opentelemetry-exporter-otlp-json-common' package

* add initial unit tests for 'opentelemetry-exporter-otlp-json-common'

* update CHANGELOG.md

* add ProtoJSON compatability tests

* add basic set of benchmarks

* fix lint errors and regenerate workflows

* fix type errors

* add type checking for opentelemetry-exporter-otlp-json-common

* remove redundant comment

* tweak comments in tests

* update classifiers in pyproject.toml

* update github workflows

* update eachdist.ini

* remove Python 3.9

* fix typechecking errors

* add 'assert_never' checks for unreachable branches

* update copyright headers