ssl: expose the negotiated TLS-1.3 group in connection_information

[jbevemyr]

The named group of the TLS-1.3 key exchange is negotiated via the supported_groups and key_share extensions and is already kept in the connection state (session.ecc), but connection_information/1,2 never exposed it: the existing ecc item only covers pre-TLS-1.3 named-curve cipher suites.

With the arrival of post-quantum hybrid groups (x25519mlkem768 and friends, OTP 28) the negotiated group has become operationally interesting: an application or an operator auditing a fleet wants to know whether a given connection actually negotiated a quantum-safe key exchange or fell back to a classical group. Today that is only visible through packet inspection or debug-level handshake logging (and for hybrid groups, openssl s_client prints a "Negotiated TLS1.3 group" line for the same reason).

This adds a selected_group item, returned for TLS-1.3 connections on both the client and the server side:

{ok, [{selected_group, x25519mlkem768}]} =
    ssl:connection_information(Socket, [selected_group]).

The item is absent for pre-TLS-1.3 connections (where ecc retains its existing semantics), and absent when no group was negotiated. Functionally verified against both a hybrid-first and a classical-only peer; ssl_api_SUITE:connection_information extended to assert the item on both sides for TLS-1.3 and its absence otherwise.

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// Erlang/OTP Github Action Bot

[jbevemyr]

Verification done on this branch, built from source on x86_64 Linux:

• ssl_api_SUITE -case connection_information passes (1 ok, 0 failed) — the case now asserts selected_group membership in ssl:groups() for TLS-1.3 and its absence otherwise.
• Both behavior branches checked directly against the built runtime: a TLS-1.3 connection reports {selected_group, ...} on both the client and the server side (also present in the connection_information/1 full list), a TLS-1.2 connection reports nothing and the pre-existing ecc item semantics are untouched.
• Also verified with post-quantum hybrid groups on OTP 28 (x25519mlkem768 negotiated and reported), which is the motivating use case.
• ./otp_build check --no-tests --no-docs --no-format-check: dialyzer passes, license-header scan 0 warnings in 11963 files. (--no-format-check because the patch has no C changes; the license check's git-history comparison step needs a full clone, the scan itself is clean.)