python: allow {id.f58plain} to be used in output formats

Problem: The f58plain encoding of jobids could be useful in some
situations with flux-jobs and other tools that support {id.<encoding>}
output format, but this encoding is currently not available since
it does not appear in `job_fields_to_attrs()` or the JobInfoFormat
headings dict.

Add `id.f58plain` where necessary to make it available as an option
in flux-jobs etc.

Author: grondo

src/bindings/python/flux/job/info.py

@@ -641,6 +641,7 @@ def job_fields_to_attrs(fields):
         "id.dec": (),
         "id.hex": (),
         "id.f58": (),
+        "id.f58plain": (),
         "id.emoji": (),
         "id.kvs": (),
         "id.words": (),
@@ -736,6 +737,7 @@ class JobInfoFormat(flux.util.OutputFormat):
         "id.dec": "JOBID",
         "id.hex": "JOBID",
         "id.f58": "JOBID",
+        "id.f58plain": "JOBID",
         "id.emoji": "JOBID",
         "id.kvs": "JOBID",
         "id.words": "JOBID",