Add an API to view current scheduled jobs

[ftianli-amzn]

Issue #, if available:
#73
Description of changes:

(This is a draft, and some details are going to be written down.)
Create a new API to get the information for all the in-memory jobs in every job index:

GET /_opendistro/_jobscheduler/{nodeId}/jobs
GET /_opendistro/_jobscheduler/jobs

The jobs information contains:

• The last execution time
• The expected next execution time
• The actual duration (in milliseconds) to next execution time

The implementation is Inspired by the stats API in ODFE Alerting plugin (https://github.com/opendistro-for-elasticsearch/alerting/blob/master/core/src/main/kotlin/com/amazon/opendistroforelasticsearch/alerting/core/resthandler/RestScheduledJobStatsHandler.kt)

The structure of the response is:

Response Body
+--- _nodes : Contains statistics about the number of nodes selected by the request. 
+--- cluster_name : Name of the cluster.: 
+--- nodes : Contains the job information for the nodes selected by the request.
         +--- Node ID
         |        +--- Job Index Name
         |        |        +--- Job ID
         |        |        |      +---  last_execution_time : The actual last running time of the job. (Unix time)
         |        |        |      +---  next_execution_time : The expected next running time of the job. (Unix time)
         |        |        |      +---  duration_to_next_execution_in_ms : The actual duration to next job runs
         |        |        \--- Job ID 2
         |        \--- Job Index Name 2
         \---Node ID 2
...

Samples:
When there is no scheduled job:

{
    "_nodes": {
        "total": 1,
        "successful": 1,
        "failed": 0
    },
    "cluster_name": "integTest",
    "nodes": {
        "PPgOgvJkRf-agf7SXAkgIw": {
            ".scheduler_sample_extension": {}
        }
    }
}

When there are multiple scheduled jobs:

{
    "_nodes": {
        "total": 1,
        "successful": 1,
        "failed": 0
    },
    "cluster_name": "integTest",
    "nodes": {
        "PPgOgvJkRf-agf7SXAkgIw": {
            ".scheduler_sample_extension": {
                "kibana-job-01": {
                    "last_execution_time": 1603240231680,
                    "next_execution_time": 1603240291675,
                    "duration_to_next_execution_in_ms": 59409
                },
                "kibana-job-02": {
                    "next_execution_time": 1603240243523,
                    "duration_to_next_execution_in_ms": 11257
                }
            }
        }
    }
}

Todo: unit tests

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ftianli-amzn]

I'm thinking if it's better to copy all the functionality in the stats API of Alerting to here.