feat(helm): add sslCertDir parameter

[keliansb]

Description

Add trivy.sslCertDir parameter to Helm chart to mount a host path containing CA certificates into the container (for example /etc/ssl/certs).

The same behavior is already available in the trivy-operator Helm chart.

This PR fixes the following error when using a vulnerability DB hosted on a private registry with a certificate signed by a custom CA :

2025-10-22T09:25:42Z    INFO    Adding schema version to the DB repository for backward compatibility    repository="registry.example.com/proxy.ghcr.io/aquasecurity/trivy-db:2"
2025-10-22T09:25:42Z    INFO    Redis scan cache    url="redis://trivy-redis:6379"
2025-10-22T09:25:42Z    INFO    [vulndb] Need to update DB
2025-10-22T09:25:42Z    INFO    [vulndb] Downloading vulnerability DB...
2025-10-22T09:25:42Z    INFO    [vulndb] Downloading artifact...    repo="registry.example.com/proxy.ghcr.io/aquasecurity/trivy-db:2"
2025-10-22T09:25:42Z    FATAL    Fatal error    failed to download vulnerability DB: OCI artifact error: failed to download vulnerability DB: failed to download artifact from registry.example.com/proxy.ghcr.io/aquasecurity/trivy-db:2: OCI repository error: 2 errors occurred:
    * Get "https://registry.example.com/v2/": tls: failed to verify certificate: x509: certificate signed by unknown authority
    * Get "https://registry.example.com/v2/": tls: failed to verify certificate: x509: certificate signed by unknown authority

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

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