Add support to pass environment variable by reference

[uristernik]

Requirements

• [ ] I have added test coverage for new or changed functionality
• [x] I have followed the repository's pull request submission guidelines
• [x] I have validated my changes against all supported platform versions

Related issues

https://github.com/launchdarkly/ld-relay-helm/issues/33

Describe the solution you've provided

Add support to pass environment variable by reference (directly passing environment variables to container)

Describe alternatives you've considered

Couldn't do it any other way

Additional context

Needed to inject environment variable by reference

[keelerm84]

Thank you for putting together this PR. I will try to take a look at this by EOD tomorrow.

[keelerm84]

Going to merge this into a local branch instead of main so I can introduce a couple of additional tweaks first.

Thank you again for this contribution!