rfc20: add optional execution.properties key to Rv1

Problem: Rv1 does not support a method for assigning properties to
resources, but simple property support is a near term requirement for
Flux.

Add an optional properties key to the Rv1 execution dictionary which
allows for the simple assignment of named properties to sets of ranks.

Slightly restrict the set of valid characters in property names.

Reserve the '@' character and any suffix for scheduler-specific use.

Co-authored-by: Jim Garlick <[email protected]>

Author: grondo

spec_20.rst

@@ -128,8 +128,9 @@ Execution
 ~~~~~~~~~
 
 The value of the ``execution`` key SHALL contain at least the keys
-``R_lite``, and ``nodelist``, with optional keys ``starttime`` and
-``expiration``. Other keys are reserved for future extensions.
+``R_lite``, and ``nodelist``, with optional keys ``properties``,
+``starttime`` and ``expiration``. Other keys are reserved for future
+extensions.
 
 ``R_lite`` is a strict list of dictionaries each of which SHALL contain
 at least the following two keys:
@@ -162,6 +163,24 @@ contain a string in RFC 29 *Hostlist Format*, e.g. ``host[0-16]``.
 
 The ``execution`` key MAY also contain any of the following optional keys:
 
+**properties**
+   The optional properties key SHALL be a dictionary where each key maps a
+   single property name to a RFC 22 idset string. The idset string SHALL
+   represent a set of execution target ranks. A given execution target
+   rank MAY appear in multiple property mappings. Property names SHALL
+   be valid UTF-8, and MUST NOT contain the following illegal characters:
+
+   ::
+
+      ! & ' " ^ ` | ( )
+
+   Additionally, the ``@`` character is reserved for scheduler specific
+   property use. Any suffix that appears after the ``@`` character in a
+   property name SHALL be a scheduler-specific string. For example,
+   ``amd-mi50@gpu``, ``amd-mi50`` SHALL be the property string, but a
+   scheduler MAY use the ``gpu`` suffix to perform scheduling optimization
+   for gpus of the corresponding ranks.
+
 **starttime**
    The value of the ``starttime`` key, if present, SHALL
    encode the start time at which the resource set is valid. The