aio: Add errcontext for processing I/Os for another backend

aio: Add errcontext for processing I/Os for another backend

Push an ErrorContextCallback adding additional detail about the process
performing the I/O and the owner of the I/O when those are not the same.

For io_method worker, this adds context specifying which process owns
the I/O that the I/O worker is processing.

For io_method io_uring, this adds context only when a backend is
*completing* I/O for another backend. It specifies the pid of the owning
process.

Author: Melanie Plageman <[email protected]>
Reviewed-by: Andres Freund <[email protected]>
Discussion: https://postgr.es/m/rdml3fpukrqnas7qc5uimtl2fyytrnu6ymc2vjf2zuflbsjuul%40hyizyjsexwmm