Add Support for PlainHTTP

Fixes #202

• Added PlainHTTP, which enables insecure HTTP connections for chart downloads
  • useful in specific scenarios where secure connections are unavailable.
• Extended support for PlainHTTP across all relevant operations:
  • Install (InstallAction)
  • Upgrade (UpgradeAction)
  • Pull (PullAction)
• Updated CRD schema (helm.crossplane.io_releases.yaml) to include the new PlainHTTP field in the spec.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review. There is no reviewable step in the Makefile

How has this code been tested

• manually deployed thepackage/crds/*
• deployed the examples/sample/release.yaml by adding an extra line after L17 plainHTTP: true, without any errors. Before my changes, this line would have caused an error