opentelemetry-sdk: sketch of an OpAMP integration (#4646)

* opentelemetry-sdk: sketch of an OpAMP integration

* OpAMP is optional

* Reword and add tests

* Reword

* Fix typing

* Rework error handling after comments

* Fix typecheck

* Fix tests

* Fix typo

* Rename entrypoint to pre_sdk_init_function

* Add post init hook as well

* Mock sdk initialization in opamp tests to avoid side effects

* Add changelog

* Move pre-init earlier

* Please pylint

* Apply suggestion from @xrmx

* Update opentelemetry-sdk/src/opentelemetry/sdk/_configuration/__init__.py

Co-authored-by: Aaron Abbott <aaronabbott@google.com>

* Convert to docstring

---------

Co-authored-by: Aaron Abbott <aaronabbott@google.com>