feat: FlexibleRollout should support "custom" stickiness.

[ivarconr]

The Flexible Rollout strategy supports multiple options for stickiness, used to define how we guarantee consistency for a gradual rollout.

Today it support the following options:

• default (Unleash chooses the first value present on the context in defined order userId, sessionId, random.)
• userId
• sessionId
• random

We have now extended the protocol to support any field in the unleash context. In addition It can be any of the predefined context field, in addition to any custom properties the user has defined.

This means that the "stickiness" parameter in the strategy configuration can now be any field on the context, and the SDK should use the specified context field to calculate the "hash" which determine whether the activation strategy should evaluate to true or false.

How it looks in the UI:

Edge case:

• If the specified context field is not specific the activation strategy should evaluate to false.

To guide the implementation we have:

• added new client-specifications, see pr-11
• implemented PoC in Node SDK see pr-201