Add option to automatically expand sub-items (e.g. vendor extension)

I have the following discriminated schema:

When changing the discriminator value to a new value, payload (which in this case contains the interesting stuff) is collapsed. (If I expand it, it remembers it if I change back to the same discriminator value, but it's always collapsed when first viewing it.)

It would be very useful for me to be able to specify that certain schema properties, when they are visible, should automatically expand.

For example, a vendor extension x-autoExpand: true could be set on my schema's payload so that each time ReDoc renders nodes with this extension set to true, it expands the object. Other good/better solutions may exist for the same use-case.

Note that this would also solve #1692 fairly elegantly.

The issue does not name files, tests, or entry points; start by tracing the rendering behavior when a discriminator changes and the payload becomes visible. Done means a documented option, such as the proposed vendor extension or an agreed alternative, automatically expands matching visible schema properties without breaking remembered expansion state.