Support `#[link_name]` (and other attributes?) in `#[pg_guard]` wrapper.

[thomcc]

In https://github.com/tcdi/pgx/pull/793#issuecomment-1285718887 I wanted to do

    #[pgx_macros::pg_guard]
    extern "C" {
        #[link_name = "pgx_SpinLockInit"]
        pub fn SpinLockInit(lock: *mut pg_sys::slock_t);
        // ...
    }

But couldn't because #[pg_guard] doesn't preserve attributes. It would be relatively simple to preserve them here: https://github.com/tcdi/pgx/blob/98ff0729c24f415b84814eb6c539d387146e4326/pgx-utils/src/rewriter.rs#L109-L111, although perhaps there's an argument that some should go on the parent (like #[doc]).

This is a pretty small nit, so I'm punting on it for now, but wanted to file the issue so I could come back to it later.

[thomcc]

A rough cut of this is https://github.com/thomcc/pgx/tree/attr-pg_guard. For some reason, it hits linker errors in other functions -- it's likely this approach is slightly too clever.

I have other things I wanted to work on today, so I'll come back to this later (maybe later today or tomorrow).

[workingjubilee]

As an update, this works on the top level but the wrapper still seems to wonkify the argument if any attempt to apply attributes to the arguments (like cfg, in https://github.com/tcdi/pgx/pull/800) is done.