Add environment variables from GCP secrets to cloud run job v2 workers

[Ultramann]

This PR adds the ability to create environment variables from GCP secrets in cloud run v2 prefect workers. To do so a new attribute was added to cloud run job v2 workers configurations, env_from_secrets which allows for the referencing of GCP secrets in a cloud run job v2 worker template with the following structure.

{
  "job_configuration": {
    "env_from_secrets": "{{ env_from_secrets }}"
  },
  "variables": {
    "env_from_secrets": {
      "title": "Secrets as Environment Variables",
      "description": "Environment variables to set from GCP secrets when starting a flow run.",
      "type": "object",
      "additionalProperties": {
        "type": "string"
      }
    }
  }
}

Deployments can then set environment variables from secrets via job variables, e.g.

"env_from_secrets": {
  "ENV_VAR": {
    "secret": "SECRET_NAME",
    "version": "latest"
  }
}

Closes #15406

Checklist

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[desertaxle]

Thanks for the PR @Ultramann! The implementation looks good, but I think we need an attribute for this field on CloudRunWorkerV2Variables to ensure it shows up in the UI.

Something like this should work:

    env_from_secrets: Dict[str, SecretKeySelector] = Field(
        default_factory=dict,
        title="Environment Variables from Secrets",
        description="Environment variables to set from GCP secrets when starting a flow run.",
        example={
            "ENV_VAR_NAME": {
                "secret": "SECRET_NAME",
                "version": "latest",
            }
        },
    )

[desertaxle]

LGTM!

[Ultramann]

@desertaxle Thanks for the quick review!