Add keyword field 'provider' to field mapping (#311)

Why these changes are being introduced:
* This is change is required to support the addition of the keyword field 
'provider' to the TIMDEX model.

How this addresses that need:
* Update config to include 'provider' keyword field

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/GDT-209

config/opensearch_mappings.json

@@ -303,6 +303,9 @@
         "type": "text",
         "index": "false"
       },
+      "provider": {
+        "type": "keyword"
+      },
       "publication_frequency": {
         "type": "text",
         "fields": {