Add CDK explorer VS code setting to allow selecting different path

**Description**

Currently the https://github.com/aws/aws-toolkit-vscode/pull/809 feature works when the cdk folder structure is found in the root of the workspace (or at most 1 level deep)

**Requested solution**

Add a vscode setting for the plugin to allow CDK base path changing per workspace. This allows working on more complex project folders/workspaces. While it may not show every single cdk in a workspace it will allow to use the tool via a setting.

**Alternatives**

Currently the only way is to reopen the folder and stop using a workspace that is a parent folder

**Examples**

Given the folder structure:

test_plugin_level_1
L test_cdk
L hello-cdk

- VSCode workspace opens folder (hello-cdk) containing CDK app.py: works ok


- VSCode workspace opens `hello-cdk` parent folder (test_cdk): still works


- VSCode workspace opens `test_cdk` parent folder (test_plugin_level_1): stops working


Start by reading the CDK explorer implementation associated with feature PR #809 and trace how it discovers CDK folders relative to the VS Code workspace. Add a per-workspace setting for the CDK base path, then verify that the example nested workspace structure can still be discovered through that setting.

Written by the indexing model from the issue text.