Support Statsig experiments (in addition to feature flags)

### Problem Statement

https://www.statsig.com/featureflags are already supported by https://docs.sentry.io/platforms/javascript/configuration/integrations/statsig/

But what about https://www.statsig.com/experimentation?

### Solution Brainstorm

Find some way to set experiment metadata on reports/spans.

: could be one way, but it omits some of the dynamic values. It's similar to : and searchable though (AFAICT).

### Additional Context

_No response_

### Priority

_No response_

Start by reading the existing Statsig feature-flag integration described at docs.sentry.io/platforms/javascript/configuration/integrations/statsig/. Then investigate how feature-flag metadata is attached to reports and spans, and determine an agreed representation for Statsig experiment names, groups, and dynamic values; done means experiment metadata is attached and searchable without losing relevant values.

索引モデルが issue の本文から書いたものです。