feat(alerting): Add specs for alerting namespace

Add OpenAPI specs for the Alerting plugin's REST API, covering:
- Monitor create, get, update, delete, search, and execute
- Alert get and acknowledge
- Finding search
- Destination get (deprecated, kept for backwards compatibility)
- Email account and email group get, exists, and search
- Scheduled-job stats

Update the pre-existing opendistro-to-plugins superseded operation
entries for alerting to use the same path parameter names as the new
namespace file, since the merger substitutes parameter names from
`superseded_by` when generating the legacy `_opendistro` aliases.

Closes #221

Author: xluo-aws

spec/_superseded_operations.yaml

@@ -12,7 +12,7 @@ $schema: ./json_schemas/_superseded_operations.schema.yaml
     - GET
     - POST
 /_opendistro/_alerting/destinations/email_accounts/{emailAccountID}:
-  superseded_by: /_plugins/_alerting/destinations/email_accounts/{emailAccountID}
+  superseded_by: /_plugins/_alerting/destinations/email_accounts/{email_account_id}
   operations:
     - GET
     - HEAD
@@ -22,12 +22,12 @@ $schema: ./json_schemas/_superseded_operations.schema.yaml
     - GET
     - POST
 /_opendistro/_alerting/destinations/email_groups/{emailGroupID}:
-  superseded_by: /_plugins/_alerting/destinations/email_groups/{emailGroupID}
+  superseded_by: /_plugins/_alerting/destinations/email_groups/{email_group_id}
   operations:
     - GET
     - HEAD
 /_opendistro/_alerting/destinations/{destinationID}:
-  superseded_by: /_plugins/_alerting/destinations/{destinationID}
+  superseded_by: /_plugins/_alerting/destinations/{destination_id}
   operations:
     - GET
 /_opendistro/_alerting/monitors:
@@ -48,34 +48,34 @@ $schema: ./json_schemas/_superseded_operations.schema.yaml
   operations:
     - GET
 /_opendistro/_alerting/monitors/{monitorID}:
-  superseded_by: /_plugins/_alerting/monitors/{monitorID}
+  superseded_by: /_plugins/_alerting/monitors/{monitor_id}
   operations:
     - GET
     - HEAD
     - PUT
     - DELETE
 /_opendistro/_alerting/monitors/{monitorID}/_acknowledge/alerts:
-  superseded_by: /_plugins/_alerting/monitors/{monitorID}/_acknowledge/alerts
+  superseded_by: /_plugins/_alerting/monitors/{monitor_id}/_acknowledge/alerts
   operations:
     - POST
 /_opendistro/_alerting/monitors/{monitorID}/_execute:
-  superseded_by: /_plugins/_alerting/monitors/{monitorID}/_execute
+  superseded_by: /_plugins/_alerting/monitors/{monitor_id}/_execute
   operations:
     - POST
 /_opendistro/_alerting/stats/:
   superseded_by: /_plugins/_alerting/stats/
   operations:
     - GET
 /_opendistro/_alerting/stats/{metric}:
-  superseded_by: /_plugins/_alerting/stats/{metric}
+  superseded_by: /_plugins/_alerting/stats/{stat}
   operations:
     - GET
 /_opendistro/_alerting/{nodeId}/stats/:
-  superseded_by: /_plugins/_alerting/{nodeId}/stats/
+  superseded_by: /_plugins/_alerting/{node_id}/stats/
   operations:
     - GET
 /_opendistro/_alerting/{nodeId}/stats/{metric}:
-  superseded_by: /_plugins/_alerting/{nodeId}/stats/{metric}
+  superseded_by: /_plugins/_alerting/{node_id}/stats/{stat}
   operations:
     - GET
 /_opendistro/_anomaly_detection/detectors: