[RRT] Add mandatory custom properties support feature

[dulithsenanayake]

Purpose

This PR adds the implementation of mandatory custom properties support feature to the publisher portal. This feature can be enabled by defining server config or tenant config as below.

Server Config

[[apim.publisher.custom_properties]]
required=true
name="PropertyName"
description="Property Description"

Tenant Config
Add the configuration as shown below by navigating to the Settings -> Advanced page of the admin portal

"PropertyConfigurations": {
	"Properties": [
		{
			"Name": "PropertyName",
			"Required": true,
			"Description": "Property Description"
		}
        ]
}

This PR includes a rest API validation to the mandatory custom properties when updating the additionalPropertiesMap.

Related PRs

• UI PR for Master: [RRT] Add UI implementation for mandatory custom properties support feature apim-apps#478