fix: fully qualify Box::pin in ServiceGenerator

[MohamedBassem]

Hello 👋

Was playing with a toy sandbox implementation baked by restate, and had a struct named Box (terrible name in the rust world, I know). This, however, uncovered an unqualified call to Box::pin in the service generator. So this tiny PR fixes that for better macro hygiene.

Repro

In the examples dir, add a struct named Box in any of the files:

diff --git a/examples/counter.rs b/examples/counter.rs
index 8f85bc8..61a761e 100644
--- a/examples/counter.rs
+++ b/examples/counter.rs
@@ -1,5 +1,7 @@
 use restate_sdk::prelude::*;

+pub struct Box {}
+
 #[restate_sdk::object]
 trait Counter {
     #[shared]

and the compiler will complain:

~/repos/restate-sdk-rust/examples ❯❯❯ cargo test                                                                                                                                                                                                                                                                                          fix/macro-hygine-box ✱
   Compiling restate-sdk v0.10.0 (/Users/mbassem/repos/restate-sdk-rust)
error[E0599]: no function or associated item named `pin` found for struct `Box` in the current scope
 --> examples/counter.rs:5:1
  |
3 | pub struct Box {}
  | -------------- function or associated item `pin` not found for this struct
4 |
5 | #[restate_sdk::object]
  | ^^^^^^^^^^^^^^^^^^^^^^ function or associated item not found in `Box`
  |
  = note: this error originates in the attribute macro `restate_sdk::object` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0599`.
error: could not compile `restate-sdk` (example "counter") due to 1 previous error
warning: build failed, waiting for other jobs to finish...

After qualifying the call, it compiles and tests are passing.

[github-actions]

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[MohamedBassem]

I have read the CLA Document and I hereby sign the CLA

[tillrohrmann]

Thanks a lot for creating this PR @MohamedBassem. LGTM. Merging this now :-)