Make endpoint handle function independent of other handlers

[h7kanna]

Hi,

I started porting my services from my own restate-rust-sdk to the official SDK.

To make my transition smooth, I am just making some small restructuring (non breaking).

I do have some useful macros and other utilities like replay aware logger, which I will start slowly contribute to the official SDK.

[slinkydeveloper]

Hi,

Thanks for the contrib. I can merge this change, but I can't guarantee I won't break this API soon, as I don't consider this part of the API public nor stable :)

I plan to do a bit of internal restructuring soon-ish (in a month or two).

[h7kanna]

Hi @slinkydeveloper, that is fine, I will adapt to restructuring again.