Implement RFC 8701 GREASE for TLS ClientHello

Implement RFC 8701 GREASE for TLS ClientHello

Add client-side GREASE (Generate Random Extensions And Sustain
Extensibility) support per RFC 8701. When SSL_OP_GREASE is set,
the TLS client injects reserved 0x?A?A-pattern values into the
ClientHello to prevent ecosystem ossification caused by servers
that reject unknown values.

GREASE values are injected into:
- Cipher suites (prepended)
- Supported versions extension (prepended)
- Supported groups extension (prepended)
- Signature algorithms extension (appended)
- Key share extension (prepended, 1 zero byte)
- Two standalone extensions (one empty, one with 1 zero byte)

The implementation uses lazy-seeded random values that remain
consistent across HelloRetryRequest retransmissions. GREASE values
from server responses are rejected as illegal parameters.

Add -grease option to s_client to enable GREASE from the command line.

Closes #9660

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.foundation>
Reviewed-by: Neil Horman <nhorman@openssl.org>
MergeDate: Tue Mar 17 14:58:25 2026
(Merged from https://github.com/openssl/openssl/pull/30303)