charts/turbinia Add Turbinia VertexAI configuration

[wajihyassine]

Description of the change

Adds the option to configure the VertexAI API key for the Turbinia LLM artifact analyzer

Applicable issues

• fixes #

Additional information

Checklist

• [ X] Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• [ X] Newly added variables are documented in the values.yaml
• [X ] Title of the pull request is descriptive