feat(options): Add support for `SENTRY_SAMPLE_RATE` environment variable / `sample-rate` property (#5112)

* feat: Add support for SENTRY_SAMPLE_RATE environment variable

Add `sampleRate` to `ExternalOptions` so it can be configured via the
`SENTRY_SAMPLE_RATE` environment variable or `sample-rate` property,
matching the behavior of other sample rate options.

Fixes GH-5091
Co-Authored-By: Claude <noreply@anthropic.com>

* docs: Add changelog entry for SENTRY_SAMPLE_RATE support

Co-Authored-By: Claude <noreply@anthropic.com>

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Co-authored-by: Claude <noreply@anthropic.com>