[DO NOT MERGE] [WSTEAM1-102] Server Side Optimizely Integration

[HarveyPeachey]

Resolves WSTEAM1-102

Overall change: Adds support for new MVT experiment headers.

Code changes:

• Adds logic to the server to extract mvt headers in a consumable format, with logic to add an enabled key which cross-checks a predetermined list and inserts this data into the page data.
• Adds logic to the server to add the correct vary header values to the response for caching.
• Adds a dummy enabled experiment list.
• Enables the mvt experiment data to be made available via the request context.
• Adds experiment activation code on the client, and forces the user into the given variation.
• Adds example experiment, which can be seen on pidgin STY pages http://localhost:7080/pidgin/58228654.
• Updates and adds unit tests.

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• [x] I have assigned myself to this PR and the corresponding issues
• [ ] I have added the cross-team label to this PR if it requires visibility across World Service teams
• [ ] I have assigned this PR to the Simorgh project
• [x] (BBC contributors only) This PR follows the repository use guidelines

Testing:

• [x] Automated (jest and/or cypress) tests added (for new features) or updated (for existing features)
• [ ] If necessary, I have run the local E2E non-smoke tests relevant to my changes (CYPRESS_APP_ENV=local CYPRESS_SMOKE=false yarn test:e2e:interactive)
• [ ] This PR requires manual testing

[HarveyPeachey]

Thoughts on using the Optimizely Event API, instead of using the hefty Optimizely SDK to send events client-side? Seems super simple https://docs.developers.optimizely.com/optimizely-data/docs/event-api-getting-started? Would save 100k in the bundle size if we don't have client-side experiments enabled on the page.

[amoore108]

Is it worth creating a new ExperimentContext solely for experimentation rather than passing the values through RequestContext? Just to keep it isolated more than anything.

[HarveyPeachey]

Is it worth creating a new ExperimentContext solely for experimentation rather than passing the values through RequestContext? Just to keep it isolated more than anything.

Can do if you think it's beneficial? I passed it down via request similar to the other stuff we add to the page data

[amoore108]

Is it worth creating a new ExperimentContext solely for experimentation rather than passing the values through RequestContext? Just to keep it isolated more than anything.

Can do if you think it's beneficial? I passed it down via request similar to the other stuff we add to the page data

Its probably alright keeping it in the RequestContext the more I think about it, since it will be set by header values 👍🏼