(toolkit-lib): fromCdkAppDirectory

### Describe the feature

`toolkit.fromCdkApp()` currently requires you to pass all parameters in directly (`app`, `context`, etc)...

We should also have something like `toolkit.fromCdkAppDirectory()` where we read all the settings from `cdk.json`, just like the CLI would do.

### Use Case

Mimic CLI

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

-

### Environment details (OS name and version, etc.)

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