[FSSDK-10711] Make use of VUID as an opt-in

[mikechu-optimizely]

Summary

• Client-side use of the visitor user ID (VUID) should be opt-in
• I made some additional devX and doc improvements

⚠️ NOTE: this is not being merged to the master branch yet, but will be released. We'll need to cherry-pick later.

Test plan

• Existing tests should pass
• New tests should also pass
• FSC end-to-end tests should pass

Issues

FSSDK-10711