Add optional TLS certificate checking when talking to Proxmox

[Luzifer]

Issue #, if available: #192

Description of changes:

• Add environment variables / flags to main process to enable certificate checking / providing root-ca when talking to Proxmox
• Add secret keys for the Proxmox credential secret to enable certificate checking / providing root-ca when talking to Proxmox
• Retain default (TLS-Insecure) in order not to introduce a breaking change (RFC)

Testing performed:

[65278]

Couple of questions, PR looks good unto itself (minus the boolean cast).
On top of the questions, we're missing documentation and work on how to map secrets as files into a deployment into SSL_CERT_DIR. That could be scoped to a different PR.

[65278]

I'm okay with this going in. Do or do not implement checking on whether data["insecure"] exists at your own leisure.

The base branch was changed.

[sonarqubecloud]

Quality Gate failed

Failed conditions
72.9% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[wikkyk]

I'm ok with the barely-missed coverage% target in this case.