feat: Support loading AdditionalContext from env var

What does this MR do and why?

Adds additional context to executor, via an env var. This MR was created by taking changes in this MR and adding back the env var, which was taken out of that branch in this commit.

Related issues

gitlab-org/gitlab#558270 (closed)

How to set up and validate locally

1. Run cmd/debug-server.go
2. Start the Executor (make sure it points at the right base-path and the right server)

--goal='test'
--insecure
--debug
--workflow-id=1
--token=your-gl-token
--base-url=http://gdk.test:3000
--user-id=1
--namespace-id=1
--project-id=1000000
--feature-enabled-by-namespace-ids=1000000
--base-path=.
--server=127.0.0.1:50052
--additional-context-content='[{"Category": "test", "Content": "test content"}]'

3. Validate the StartWorkflowRequest in the debug server's log contains the expected additional context.