feat(mstsgu): add NTLM RPC association setup

[Marc-André Moreau (mamoreau-devolutions)]

Add DCE/RPC NTLM association setup for RD Gateway, including header-signing negotiation.
Keep its SSPI context independent from HTTP authentication contexts.

This excludes live RPC-over-HTTP transport and protected RPC traffic.

[Copilot]

Pull request overview

Adds NTLM-authenticated DCE/RPC association setup for RD Gateway while keeping HTTP authentication contexts separate.

Changes:

• Adds NTLM bind, bind-ack, and rpc_auth_3 codecs.
• Introduces independent SSPI association state.
• Adds wire-format and state-transition tests and documentation.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
crates/ironrdp-mstsgu/src/rpc.rs	Implements NTLM association state and authenticated RPC codecs.
crates/ironrdp-mstsgu/tests/rpc_pdu.rs	Tests wire layouts, validation, and NTLM sequencing.
crates/ironrdp-mstsgu/tests/rpc_tsgu_stubs.rs	Adds error stubs required by RPC tests.
crates/ironrdp-mstsgu/README.md	Documents the new association support.
crates/ironrdp-mstsgu/CHANGELOG.md	Records the added feature.

Suppressed comments (3)

crates/ironrdp-mstsgu/src/rpc.rs:1699

• The security trailer is required to start on a 16-byte boundary relative to the PDU body (MS-RPCE 2.2.2.11), but the decoder only subtracts the declared padding and never checks the trailer offset. A packet can therefore use a self-consistent auth_pad_length while placing the trailer at a nonconforming offset and still be accepted. Validate body_with_padding.len() % 16 == 0 before decoding the body.

    let body_with_padding = &source[RPC_COMMON_HEADER_SIZE..trailer_offset];
    let padding_length = usize::from(trailer[2]);
    // The gateway profile accepts canonical zero-filled padding and uses
    // auth_pad_length rather than a fixed bind_ack layout.
    let body_length = body_with_padding
        .len()
        .checked_sub(padding_length)
        .ok_or(RpcPduError::InvalidAuthenticationPadding { actual: trailer[2] })?;

crates/ironrdp-mstsgu/src/rpc.rs:1682

• MS-RPCE 2.2.2.11 says auth_reserved should be ignored on read. Rejecting a nonzero value makes the bind acknowledgement decoder unnecessarily incompatible with peers that populate this reserved byte. Ignore it during decoding; retaining zero on encode is correct.

    if trailer[3] != 0 {
        return Err(RpcPduError::NonZeroAuthenticationReserved { actual: trailer[3] });

crates/ironrdp-mstsgu/src/rpc.rs:1701

• The authentication padding is alignment-only; MS-RPCE 2.2.2.11 specifies its length and placement but does not require its octets to be zero. Rejecting nonzero padding imposes an extra wire constraint and can reject otherwise conforming bind acknowledgements. Strip the declared padding without inspecting its contents.

    if body_with_padding[body_length..].iter().any(|&byte| byte != 0) {
        return Err(RpcPduError::NonZeroAuthenticationPadding);