support update of job constraints

[wihobbs]

A user wrote into the hotline today asking if they could update the constraints section of their job, after realizing certain nodes on a cluster were repeatedly erroring out in MPI_Waitall. It appears we don't support updating constraints currently:

(s=130,d=0)  fluke2 ~ $ flux jobs
       JOBID QUEUE    USER     NAME       ST NTASKS NNODES     TIME INFO
 fH2sUZESArj batch    hobbs17  hostname    S      1      1      30m held
(s=130,d=0)  fluke2 ~ $ flux update fH2sUZESArj attributes.system.constraints=""
flux-update: ERROR: update of attributes.system.constraints not supported

[grondo]

flux update does support updating the queue, which requires updating constraints. So maybe this one is more tractable than #6704? My intuition is that it will be more user friendly to support replacement of the non-queue constraints in total with flux update using the same constraints parser used by the flux submit --require option, e.g.

$ flux update JOBID constraints=host:-fluke100

Would cause a constraints update plugin to:

• remove all constraints
• add the new constraint
• add the queue constraint
• validate job

If the above were successful, then the constraint update would be applied via a jobspec-update event.