feat(perf-client): env flags to suppress cache-busting headers

[michaelfeil]

Summary

The performance client emits two headers that vary per request and break header-based downstream caching (e.g. when a cache server SHA-hashes request headers as the cache key):

• Request-Deadline-Ms — time-based (now + timeout), recomputed every request (core/src/http_client.rs)
• x-baseten-customer-request-id — unique per request (UUID + batch/retry/hedge counters)

When the user has no control over the cache server, the only lever is to stop emitting these headers at the client. This adds two opt-in env flags (default behavior unchanged):

• PERFORMANCE_CLIENT_DISABLE_TIMEOUT_HEADERS — suppresses Request-Timeout-Ms / Request-Deadline-Ms
• PERFORMANCE_CLIENT_DISABLE_REQUEST_ID_HEADER — suppresses x-baseten-customer-request-id (internal retry/hedge/tracing behavior is unaffected; only the emitted header is skipped)

Truthy values: 1, true, yes, on (case-insensitive). Values are read once and cached via OnceLock to avoid a syscall per request.

Changes

• core/src/constants.rs: new env var name constants
• core/src/http_client.rs: env_flag_enabled helper + cached flags; add_timeout_headers and both CUSTOMER_HEADER_NAME injection sites are now gated
• README.md: document the two new env vars

Test plan

• cargo test -p baseten_performance_client_core (49 passed, incl. new http_client::tests*`)
• cargo clippy -p baseten_performance_client_core --all-targets -- -D warnings clean
• cargo fmt --check clean
• cargo check for python + node bindings (no public API change)

Notes

• No API/builder changes, so Python/Node bindings require no updates.
• Suppression is env-var-only per the requested design.