Expose TLS exporter keying material

[davidtorcivia]

Summary

• expose BoringSSL TLS exporter keying material through Connection
• preserve the distinction between an absent exporter context and a present context
• verify that connected peers derive the same material and that labels and contexts domain-separate outputs

The API accepts a caller-owned output buffer and does not allocate.

Validation

• cargo +nightly-2026-07-15 fmt --all -- --check
• BINDGEN_EXTRA_CLANG_ARGS=-I/usr/lib/gcc/x86_64-linux-gnu/13/include cargo +nightly-2026-07-15 test -p quiche
• BINDGEN_EXTRA_CLANG_ARGS=-I/usr/lib/gcc/x86_64-linux-gnu/13/include cargo +nightly-2026-07-15 clippy -p quiche --lib -- -D warnings

Local results: 1,090 unit tests and 45 doctests passed.