[MultistepDialog] Allow side navigation to be disabled (e.g. when submitting)

Environment

• Package version(s): @blueprintjs/core@3.48.0

Feature Request

1. Add a disabled prop to <MultistepDialog> that would allow the side navigation to be disabled (e.g when the dialog is being submitted)

2. Perhaps if specified true (and ignored as false | undefined), it could also combine & override any individual disabled props for all the other steps' nextButtonProps and backButtonProps? (You can see in my Code Sandbox example, I am passing || submitting through to most of them)

Code Sandbox

https://codesandbox.io/s/blueprint-sandbox-multistepdialog-4di0d

Steps to reproduce

(Using the example in the Code Sandbox)

1. Advance through each step of the <MultistepDialog> to the end.
2. See how each step must be "valid" before you can progress.
3. On the last step, click the "Submit" button to trigger a fake submit state.
4. ❌ See that you can still navigate around between past steps via the side navigation whilst the form submit is ongoing.