WIP: High-level overview of Pirate Metrics params & signals

[Jeehut]

🚧 This is a Work-in-Progress PR. Do not merge! 🚧

My goal with this PR was to get an high-level overview of all the things need to be done for the pirate metrics topic.

@winsmith Please have a look at this code. This is how I imagine the whole thing to look like at the end. Of course, many details are missing, including documentation and all the various TODOs.

But rather than sending you one huge PR at the end, I want you to review & approve this overview first (without merging it!). Then, I can tackle one TODO at a time and send you small PRs that finalize various aspects, which will all target this branch. When all TODOs are done, you will already have reviewed & approved all code and can take a last "overall" look if you want, before we merge it.

Let me know if you have any concerns with this approach and if you have any concerns with the overall approach I've outlined in this PR. Looking forward to your feedback! And excited for the feature. 😍

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These 14 subtasks need to be handled before we can publish a first version of "Pirate Metrics" to users:

• SwiftSDK: Persist session data with automatic start/end/duration tracking)
• SwiftSDK: Auto-detect and send new installation
• SwiftSDK: Calculate and send session-related stats like average length, distinct days used etc.
• SwiftSDK: Auto-detect free trial start + persist to auto-detect free-trial conversions
• SwiftSDK: Persist acquisition channel & send with every request
• SwiftSDK: Add documentation comments to all new public API methods
• Docs: Add all new parameters and their types to the default parameters document
• Docs: Create document describing how to send data for all Pirate Metrics insights
• Web: Create Core Insights for "Acquisition" (skip insights currently not possible)
• Web: Create Core Insights for "Activation" (skip insights currently not possible)
• Web: Create Core Insights for "Retention" (skip insights currently not possible)
• Web: Create Core Insights for "Referral" (skip insights currently not possible)
• Web: Create Core Insights for "Revenue" (skip insights currently not possible)
• Docs: Create document explaining the Pirate Metrics method and how to learn from the insights

These tasks can be tackled after the release:

• Social Media: Write posts/snippets that can be scheduled to market these new features & insights
• Web: Create tasks for each missing insight, explaining what internal functionality is missing to create it
• SwiftSDK: Add SwiftUI component for asking for NPS Score / User Rating & feedback
• Docs: Create document on NPS Score / User Rating setup & how to learn from it