Add `update_mutation` fn to `zapp g model` command

[POPPIN-FUMI]

Now zapp g model generates create_mutation and delete_mutation for mutation.

Let's add the update_mutation function!

https://github.com/EpicsDAO/zapp/blob/main/src/g/graphql/mutation.rs

[Akhil-Rawat]

@POPPIN-FUMI please assign that issue to me