Feature multiple operations webhook

[mehrdad-khojastefar]

Added support for multiple operations for one handler.
closes: #1057
related: #785

So now you can have handlers like this:
@kopf.on.validate("tempaccess", operation=["CREATE", "UPDATE"])

Also it is backward compatible so you can use it like this too:
@kopf.on.validate("tempaccess", operation="CREATE"

I have run tests using pytest and all of them passed.
I also add this commit to one of our projects and tested it manually. It successfully creates the validating webhook with multiple endpoints when managed option is set.

@nolar let me know it there is anything more that I must do. Thanks.

[SmsS4]

This PR would help us in our web-hooks to remove duplication in our codebase.

[nolar]

Thanks for sending this PR! I have made a few adjustments:

list[] is a syntax from Python 3.9+, while Kopf has to support at least Python 3.8 (I will drop 3.7 in #1062 now). It should be List[], not list[] from PEP-585.

Besides, I suggest it to be any collection, be that a list, a tuple, a set, a frozen set, or something self-made. Hence, Collection[] as the most generic class with size & items but no ordering (which would make it a Sequence[]).

And also I prefer to have clear names about the content of that field, so it should be in plural, not singular. I have added operations (plural) everywhere and made operation (singular) supported but deprecated (with a warning — as for some other deprecated names, e.g. namespace/namespaces).

The Operations type is removed as too verbose — it can be expressed with StdLib's typing library + Operation, does not contain complexity like Union[], so let's keep it simple.

[mehrdad-khojastefar]

I can update the docs and examples as well on weekend.