Executor grabs model metadata information from ENV variable and sends it as a header to DWS

What does this MR do and why?

First step towards gitlab-org/gitlab#575859 (closed), which supports Duo Workflow Executor to:

• grab a variable named AGENT_PLATFORM_MODEL_METADATA from the environment variables
• use this variable, and set the value of the variable as the value of thex-gitlab-agent-platform-model-metadata header that is sent to the Duo Workflow Service

This change adds support for sending model metadata information when making requests to GitLab's API. The code now accepts a new configuration parameter called "model-metadata" that can be set via command line or environment variable, and this metadata gets included as a special header in all API requests. This allows the system to communicate details about which AI model is being used (like "gpt_5" from GitLab) when processing workflows, which helps with tracking and analytics on the server side.

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