[FEATURE] Create Datasource API.

[vmmusings]

Create a new API for creating datasource.
Example for new creation API:

POST API : _plugins/_query/_datasources

{
        "name" : "my_prometheus",
        "connector": "prometheus",
        "properties" : {
            "prometheus.uri" : "http://localhost:8080",
            "prometheus.auth.type" : "basicauth",
            "prometheus.auth.username" : "admin",
            "prometheus.auth.password" : "admin"
        },
        "allowedRoles" : ["prometheus_access"]
 }