Support for multiple pages sized experiment

[mfendeksilverstripe]

Description

Currently, it's only possible to set up an experiment on a single page. This can be limiting as some experiments may want to span multiple pages. The bit that's missing here is a shared cookie across these pages so the content variation choice becomes consistent within the whole scope of the experiment.

Additional context or points of discussion

No response

Validations

• You intend to implement the feature yourself
• You have read the contributing guide
• You strongly believe this feature should be in core, rather than being its own community module
• You have checked for existing issues or pull requests related to this feature (and didn't find any)