C# bindings for scheduled tables

[Shubham8287]

look at corresponding rust binding ticket - #1326.

modify spacetimedb(table) macro for scheduled argument as described here - https://github.com/clockworklabs/SpacetimeDBPrivate/blob/shub/timers-proposal/proposals/0005-timers.md#api-design

[RReverser]

I'll get around to implementing this after the refactoring I'm currently working on, but meanwhile @Shubham8287 would it be possible to extend the spec with proposed API for C#? (you can take a look at C# module reference docs for how it works now)

I'm happy to implement it, but if you could spec out the design, that would be most helpful.

Going forward, perhaps we should always declare design for all languages in proposals, e.g. like I did in the Consistent Filtering proposal.

[bfops]

I believe this is done (in #1449)