flux-job: support MPIR_executable_path and MPIR_server_arguments in attach

[grondo]

Problem: flux job attach MPIR support does not include support for MPIR_executable_path and MPIR_server_arguments which specify when and how to launch tool daemons alongside the job being debugged.

Ref: https://www.mpi-forum.org/docs/mpir-specification-03-01-2018.pdf

9.15 MPIR_executable_path
Global variable definition

char MPIR_executable_path[256]
| Definition is not required.
| Definition is contained within the address space of the starter process.
| Variable is written by the tool, and read by the starter process.

MPIR_executable_path is a null-terminated character string that is written by the tool into the address space of the starter process. The string is the path name of the tool daemons executable file on the target node. When the tool then sets MPIR_being_debugged to a non-zero value and continues the starter process, the starter process notices that the value of MPIR_being_debugged changed to a non-zero value, and launches the executable named by the MPIR_executable_path variable and passes the arguments contained in the MPIR_server_arguments variable.

[grondo]

Closed by #5758