feat: allow setting env vars for hydra-maester

[ariep]

Add a deployment.extraEnv value to the hydra-maester helm chart for setting environment variables.

Related Issue or Design Document

Fixes #650.

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact [email protected]) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

I followed the example of the hydra chart, using the same variable name.

[Demonsthere]

lgtm! :)