fix: variant stickiness from strategy

[Tymek]

Description

Updated to the latest client-specification. In a specific scenario, when stickiness is available on strategy but not on variants themselves, we should use it.

https://github.com/Unleash/client-specification/pull/83 https://github.com/Unleash/client-specification/commit/89f7bc68f84ef04311d65582b6119837646f3270

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules

[Tymek]

Can we put a proper description on this? I don't really know what this is or why it needs to be done without rooting through that version of the client spec

Good point. Added, with links to relevant PR and commit