Command Line Flag: --approve-training-data

### Describe the feature or problem you'd like to solve

I am happy to let you train on most of my prompts, but it needs to be opt-in for security. This way I can toggle my willingness to provide training data on the fly.

### Proposed solution

For a variety of important reasons, I'm unable to leave training data "on" as my default setting. It must be off by default for security's sake with my toolchain. It would however be trivial for me to add a flag to most of my copilot calls so that I can submit them for training data.

I would like to support the further development of various LLMs but I need a way to quickly toggle it on or off for a chat session.

Acceptance criteria should probably include some kind of separation between secured sessions and unsecured sessions. Perhaps the secure sessions live in your user .copilot directory which should usually be outside the scope of a project folder. That may be the default case already.

### Example prompts or workflows

`copilot --approve-training-data`

### Additional context

_No response_