OTel: Sentry

[khvn26]

1. Move over Sentry SDK initialisation from flagsmith/flagsmith to here.
2. Port https://github.com/Flagsmith/flagsmith/blob/main/api/integrations/sentry/samplers.py to a opentelemetry.sdk.trace.sampling.Sampler subclass, and maintain compatibility with existing Sentry tracing settings DEFAULT_SENTRY_TRACE_SAMPLE_RATE, DASHBOARD_ENDPOINTS_SENTRY_TRACE_SAMPLE_RATE, FORCE_SENTRY_TRACE_KEY. Introduce non-sentry aliases for these settings (e.g. OTEL_FORCE_TRACE_KEY). Add the Sampler to OTel tracing setup.
3. Initialise Sentry SDK with integrations=[OTLPIntegration()].
4. Make sure Sentry tracing, breadcrumbs, and error capture work.