feat: Custom stickiness for variants

[ivarconr]

This is kind of similar to #71, but for variants.

The stickiness will be defined as a field on all the variants in the variants array, this to ensure we are not breaking our protocol. In the example we can see the stickiness is configured to "tenantId".

{"variants": [
     {
		"name": "yellow",
		"weight": 166,
		"weightType": "variable",
		"payload": {
			"type": "string",
			"value": "yellow submarine"
		},
		"overrides": [],
		"stickiness": "tenantId"
	}, {
		"name": "salmon",
		"weight": 166,
		"weightType": "variable",
		"payload": {
			"type": "string",
			"value": "salmon"
		},
		"overrides": [],
		"stickiness": "tenantId"
	}
]}

How it looks in Unleash Admin UI:

Edge cases:
If no stickiness is defined the SDK should assume "default" stickiness as today.

To guide the implementation we have:

• added new client-specifications, see pr-11
• implemented PoC in Node SDK see pr-202