Enable SSL configuration in application.yaml

[Haribaskar1018]

Fixes #58

Added SSL configuration options under server.ssl in application.yml,
following the same environment-variable pattern used elsewhere in
this file (e.g. MYSQL_USERNAME, MYSQL_PASSWORD). SSL is disabled by
default so existing deployments are not affected.

[Haribaskar1018]

Hi maintainers, thanks for reviewing! Note: the failing build-and-push
checks appear related to Docker registry credentials not being available
to fork PRs, not the code change itself. Happy to make any adjustments
you'd suggest.