Entity Isolation proposal refreshed (to reflect implementation)

[Keith W]

I let the Entity Isolation proposal (https://github.com/kroxylicious/design/pull/80) rot somewhat whilst work on its implementation took the lead.  It turned out there was far more to the problem than was appreciated.

The first version of the Entity Isolation is delivered and will be part of 0.20.0.

I've retrospectively made updates to the proposal to reflect what we actually did.

https://github.com/kroxylicious/design/pull/80

We stepped back from implementing topic isolation.  We've documented what makes topic isolation hard  and the challenges that need to be over-come in  https://github.com/kroxylicious/kroxylicious/issues/3504.

Thanks Keith.