[Mobile Vitals] Time-to-initial-display & initial concept for time-to-full-display

[aritchie]

This adds a bunch of additional needed points

• Finish(DateTimeOffset) added to ISpan members. Marked as default nullable to prevent breaking signature
• event StatusChanged for ISpan to allow monitoring of all spans to await finish
• Adds a default DateTimeOffset for setting startTimestamp on through ISentryClient and span ctors
• A new MAUI extension point in IMauiPageEventHandler
  • This may be marked as internal since I don't think we should open this up to external users
  • I wanted to separate this from IMauiElementEventBinder since it has several different hooks I wanted to watch

There are a few TODOs on this, but I wanted to get this in for initial thoughts

TODO

• Unit Tests
• Feature Flag?
• Additional logic for TTFD

[github-actions]

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