fix operation mixins across multiapi versions with different signatures

[iscai-msft]

Current design example

Overview. These steps are only added for operations that have different signatures. Rest are left untouched:

1. I make the signature take in the union of all possible parameters across api versions
2. I validate the parameters per api version through a decorator
3. Then I enter the mixin as i regularly would.
4. Return mixin call based on api_version

[iscai-msft]

Here's another design I came up with using "overloads": #697

Closing this issue since we've decided that we can get around it for now (only seeing this in key vault). May continue looking into this issue if we see a valid case in the future (i.e., one that can't be gotten around by not flattening / asking the service team to change the name of the method)

[lmazuel]

Known multi API mixin mgmt:

• Network
• Webapp

So as long as it works for then, low-pri. If they have this issue, then shift to required for mgmt GA.