add typescript definition for Oneof fields

[yangjian]

ATT

[stanley-cheung]

Sorry for the late look at this. There had been some work to re-sync this repo with our internal codebase so it looks like there are some new conflicts.

Can you please resolve them and rebase to the latest master please? Thanks

[yangjian]

Just resolved conflicts, and rebased to latest master. Please let me know if there is any issue in the change.

[stanley-cheung]

Ah, sorry, some more conflicts after a merge. Please rebase again. This PR will be next.

[yangjian]

Just resolved conflicts, and rebased again.

[stanley-cheung]

Thanks for your contributions!

[rogchap]

I have a weird side effect of this: it seems the hasXXX() method is being defined twice; our example:

getLogicfield(): LogicfieldTemplateItem | undefined;
setLogicfield(value?: LogicfieldTemplateItem): void;
hasLogicfield(): boolean;
clearLogicfield(): void;
hasLogicfield(): boolean;

getItemCase(): TemplateItem.ItemCase;

[yangjian]

Sorry. I didn't noticed that this method has be defined during resolving conflicts: https://github.com/grpc/grpc-web/blob/master/javascript/net/grpc/web/grpc_generator.cc#L863

submitted PR #519 to fix this issue.