Implement `ctx.Identity` getter in C# modules

[RReverser]

Analogous to the ctx.identity() method in Rust, as implemented in SpacetimeDB#2085.

TBH I forget whether we specified this as a method or an attribute in C#, but whatever. Either way, the getter makes a call to the identity host function (added in the above PR), treats its result as a little-endian Identity, and returns that Identity. This should take all of 15 minutes to implement, hopefully.

[RReverser]

Ugh this is really dumb that Github converts draft issues to issues, but now with the person who did the conversion as the author...

@gefjon I think this was initially created by you? To clarify, ctx.Identity is different than the existing ctx.CallerIdentity? Should we maybe name it ctx.ModuleIdentity instead? Just "identity" seems potentially ambigous, I can see how someone might confuse the two.

[RReverser]

Also, can't this be a standalone function or at least a static variable since it doesn't depend on the ReducerContext in any way?

[gefjon]

This is designed to be analogous to the same method on clients. Any SpacetimeDB actor, client or module, should be able to write the same code snippet to access its identity. Thus ctx.identity() or ctx.Identity. In clients it should not be called ModuleIdentity, and cannot be a free function or global variable.

I believe @cloutiertyler is going to request that we revert ctx.CallerIdentity to be ctx.Sender in C#, which will reduce confusion.

[RReverser]

All good points, thanks.