crypto/tls: omit PSK in ECH outer client hello #79282

The Encrypted Client Hello implementation would leak the pre-shared key
identities during the handshake, allowing a passive network observer who can
collect handshakes to de-anonymize the hostname of the server, even when ECH was
being used.

Thanks to Coia Prant (github.com/rbqvq) for reporting this issue.

This is CVE-2026-42505 and Go issue https://go.dev/issue/79282.

This was a PUBLIC track issue, tracked in http://b/490043318.