Add disposable utilities (possibly as a "separate" library?)

[nadako]

When working with lifetime helper and embracing IDisposable it's often needed to have composite disposables, dummy empty disposables, callback disposables, cancellation-token disposables, event unsubscription disposables, disposable proxies and so on.

I normally just have what I need in specific project but this is a good candidate for the library code. It's not specifically related to TinkState# and state handling tho, so it might make sense to have a separate "lifetime helper" library with all this (also move Unity extensions there).

[nadako]

Did some initial work in a separate repo here: https://github.com/nadako/DisposeHeaven