Add a config source for env vars

[pjanotti]

Although the OTel collector already supports env variables on the config for feature parity with SA we need to add a full config source for env vars. Having env vars as a config source allows injection of YAML fragments, like SA, via environment variables. Added also the following:

• Possibility to define defaults/fallbacks when environment variables used in the config are undefined in the environment.
• A retrieve parameter to indicate that the configuration load should fail if the environment variable is not defined.

[pjanotti]

This likely can be better handled by the config source manager. I will evaluate that option and will update after merging this PR if appropriate. For now, the desired behavior is enforced here.