GH-534: Add additive Solr modules env and args support

[tormi]

Summary

• Add additive Solr module configuration via solr.modules rendered as SOLR_MODULES.
• Add additive Solr env injection via solr.extraEnv (supports map and env-list forms).
• Add additive Solr args via solr.extraCommandArgs appended after default solr.commandArgs.
• Extend chart schema to allow the new Solr keys.

Validation

• Local helm template render checks for Solr env and args output.
• Verified both solr.extraEnv forms (map and env-list).

Closes #534

[ArtisKrumins]

@tormi What is added value versus just using extra flag Dsolr.modules=module1,module2 ?
SOLR can have various additional flags and I would suggest to stick to one method instead of adding functionality for each case.