feat!(core): Add event sampling strategy

[szokeasaurusrex]

Replace the public ClientOptions::sample_rate field with an EventSamplingStrategy enum while preserving the existing sample_rate(...) builder API for fixed-rate event sampling.

This mirrors the traces sampling model added in #1227 and unblocks #1127 by making it possible to add a sampling function variant to EventSamplingStrategy without another public field replacement.

BREAKING CHANGE: Remove the public ClientOptions::sample_rate field. Use ClientOptions::event_sampling_strategy to inspect event sampling configuration, and continue using ClientOptions::sample_rate(...) to configure a fixed sample rate.

References #1127
References #1127

[linear-code]

RUST-215

[cursor]

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[github-actions]

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