Early Draft specs doc for DatabaseLoader in ML.NET (#3857)

* Early Draft specs doc for DatabaseLoader in ML.NET

* Specifying implementaion as .NET Standard 2.0

* Support connect from within the RDBMS server

* Updated changes related to embeded connection

* Updates on Azure SQL DB support

* Minor updates on SQL CLR

* Minor typo fixed

* Refactored some code naming

Author: CESARDELATORRE