Support partial list update (append) in state manager

[bushig]

What's New

• Added smarter handling of array operations with state management. Now when you append to arrays, Beanie uses MongoDB's $push operator rather than replacing the entire array.
• Also some changes to docs to make cleaner why one would use state manager.
• fixture to check what operations was actually sent to Mongo by motor, instead of using get_changes.

More details

• Array append() operations now generate MongoDB $push operations with state_management=True
• Only happens when state_management_replace_objects=False (which is the default)
• Other array operations (changing items, removing them) still work as before, cause they unsafe for sure.
• The get_changes() method still shows the full array for compatibility

Also thinking about adding option for unsafe direct array operations like changing elements by index, but still evaluating if its worth it.

[bushig]

Fixed tests, but there is still two failing because of docker hub image pull limits and one seems to be flaky test
Can anyone rerun failing tests?

[staticxterm]

Hi @bushig, thanks for the PR.
I'll rerun the tests. Which reminds me that I should push some PR that would hopefully fix those flaky tests.
When I get some free time, I'll take a look at the PR.