Separate `tox.ini` into multiple files

[ocelotl]

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Fixes #2478

This is still a draft PR because I have not yet refactored all packages so that all of them have their own tox.ini file.

The advantages of this proposal are:

1. The main tox.ini file will be much smaller. Right now, this file is extremely big.
2. The tox commands will be much simpler. Instead of running tox -e py38-test-opentelemetry-name-of-the-package we will be able to just cd into the package directory and then run tox -e test-py38.
3. This PR will add coverage testing for all packages.
4. Right now we run pytest with the benchmark options active for all tests, even when only one package here has benchmark tests. This introduces a warning in all of our logs. We should only run pytest with the benchmark options when there are benchmark tests, this PR fixes this issue.
5. Right now, we should not run coverage nor benchmark tests in CI (please read the comments below for an explanation). This PR allows us to not run these tests in CI.
6. Having a separate tox.ini file per package allows users to simply run tox in the package they changed to run all necessary tests.