Add a max number of variants per POST for HGVSg

[zhx828]

Annotating HGVSg uses a lot of resources. If a request contains too many variants, it will occupy the thread and further drags down the service. We should put a cap at 200.

May be worth to add a threshold for other annotation endpoints too.

[zhx828]

We are working on improving the GN performance, hopefully this is not really needed.