Add support to configure multiple GraphTermServiceProvider instances

[JohnTimm]

In order to flexibly support user-defined segmentation of code system content across one or more graph databases and for consistency with RemoteTermServiceProvider we would like to allow for multiple configuration elements in fhir-server-config.json as follows:

            "graphTermServiceProviders": [{
                "__comment": "Configuration for Berkeley DB Java Edition + Lucene",
                "enabled": false,
                "configuration": {
                    "storage.backend": "berkeleyje",
                    "storage.directory": "data/graph",
                    "index.search.backend": "lucene",
                    "index.search.hostname": "data/searchindex",
                    "storage.read-only": true,
                    "query.batch": true,
                    "query.batch-property-prefetch": true,
                    "query.fast-property": true
                }
            },{
                "__comment": "Configuration for Apache Cassandra + ElasticSearch",
                "enabled": true,
                "timeLimit": 10000,
                "configuration": {
                    "storage.backend": "cql",
                    "storage.hostname": "127.0.0.1",
                    "index.search.backend": "elasticsearch",
                    "index.search.hostname": "127.0.0.1:9200",
                    "storage.read-only": true,
                    "query.batch": true,
                    "query.batch-property-prefetch": true,
                    "query.fast-property": true
                }
            }],

[lmsurpre]

When I started my server with the old config, it failed with a NPE. We need to handle the absence of config parameters just as well as their presence.

[lmsurpre]

I opened a PR to address this config issue and that has been merged.