[cirqflow] Provide concrete ExecutableSpec (#4559)

`ExecutableSpec` is a newly-introduced abstract class. The design was each application would subclass it for application-dependent data attributes. 

1. Writing all the serialization tests is a nightmare without a full-fledged lives-in-Cirq concrete implementation of this
2. You could imagine users might want a lower-touch way to put together an executable using an untyped, implicit-schema version of the spec.

Ergo: this PR introduces `KeyValueExecutableSpec` to address these two concerns.

Author: mpharrigan

cirq-google/cirq_google/__init__.py

@@ -120,6 +120,7 @@
 
 from cirq_google.workflow import (
     ExecutableSpec,
+    KeyValueExecutableSpec,
     QuantumExecutable,
     QuantumExecutableGroup,
     BitstringsMeasurement,

cirq-google/cirq_google/json_resolver_cache.py

@@ -44,4 +44,5 @@ def _class_resolver_dictionary() -> Dict[str, ObjectFactory]:
         'cirq.google.BitstringsMeasurement': cirq_google.BitstringsMeasurement,
         'cirq.google.QuantumExecutable': cirq_google.QuantumExecutable,
         'cirq.google.QuantumExecutableGroup': cirq_google.QuantumExecutableGroup,
+        'cirq.google.KeyValueExecutableSpec': cirq_google.KeyValueExecutableSpec,
     }

cirq-google/cirq_google/json_test_data/cirq.google.KeyValueExecutableSpec.json

@@ -0,0 +1,10 @@
+{
+  "cirq_type": "cirq.google.KeyValueExecutableSpec",
+  "executable_family": "cirq_google.algo_benchmarks.example",
+  "key_value_pairs": [
+    [
+      "name",
+      "example-program"
+    ]
+  ]
+}

cirq-google/cirq_google/json_test_data/cirq.google.KeyValueExecutableSpec.repr

@@ -0,0 +1 @@
+cirq_google.KeyValueExecutableSpec(executable_family='cirq_google.algo_benchmarks.example', key_value_pairs=(('name', 'example-program'),))