Toolkit should autoinstall CDK CLI

## Problem

Installing CDK CLI is an extra step.

Related: https://github.com/aws/aws-toolkit-vscode/issues/3028

## Expected behavior

- If Toolkit doesn't find CDK CLI on the system, offer to auto-install it.
- Also need to auto-update it.
- The installed copy will be "private" to AWS Toolkit, it won't pollute the customer's system $PATH.
- Problem: much of `cdk` cli usage involves user invocations of `cli` in the terminal, so if Toolkit intalls a "private" copy we would need to make it available in vscode terminal somehow.

Start by reviewing related issue #3028 and the expected behavior in this issue. Define how Toolkit detects, installs, updates, and exposes a private CDK CLI copy in the VS Code terminal; the work is done when those behaviors and the PATH implications are resolved.

Written by the indexing model from the issue text.