Feature request: allow passing the full OpenAPI config to the serverless runtime image as an environment variable

I'd like to request allowing passing the full value of the ~~OpenAPI~~ Service Management config as an environment variable, rather than just setting the path to the config as an environment variable.

Example:
```bash
gcloud run deploy ... \
--image="gcr.io/endpoints-release/endpoints-runtime-serverless:2" \
--set-env-vars=ENDPOINTS_SERVICE_CONFIG="{ ... }" \
```

This has several advantages, most notably that:
- The step of building your own image is eliminated.
- Updating the configuration becomes much faster because you only need to change an environment variable.
- Overall simplification.

I also think you don't have to get rid of the old configuration methods, you can just add this as another option, similar to how both `ENDPOINTS_SERVICE_NAME` and `ENDPOINTS_SERVICE_PATH` are still available.

Start from the serverless runtime image's handling of ENDPOINTS_SERVICE_CONFIG, using the deployment example in the issue as the entry point. Determine how a full Service Management configuration can be accepted while retaining ENDPOINTS_SERVICE_NAME and ENDPOINTS_SERVICE_PATH; done means the environment variable works without building a custom image and the existing methods remain supported.

Written by the indexing model from the issue text.