feat(client): support gssapi-with-mic auth

[ayamir]

Summary

Add client-side support for the RFC 4462 gssapi-with-mic userauth method. This keeps russh responsible for the SSH protocol flow while leaving platform GSSAPI/Kerberos integration to callers through a new GssapiAuthenticator trait. Callers provide DER-encoded mechanism OIDs, consume server GSS tokens, and return continuation tokens or the final MIC.

Changes

• Add MethodKind::GssapiWithMic / Method::GssapiWithMic
• Add RFC 4462 GSSAPI userauth message constants:
  • USERAUTH_GSSAPI_RESPONSE = 60
  • USERAUTH_GSSAPI_TOKEN = 61
  • USERAUTH_GSSAPI_EXCHANGE_COMPLETE = 63
  • USERAUTH_GSSAPI_ERROR = 64
  • USERAUTH_GSSAPI_ERRTOK = 65
  • USERAUTH_GSSAPI_MIC = 66
• Add GssapiAuthenticator and GssapiStep
• Add Handle::authenticate_gssapi_with_mic
• Handle server GSSAPI response/token/error/errtok packets in the client auth state machine
• Encode client GSSAPI token, MIC, and exchange-complete packets
• Add protocol-level tests for:
  • method parsing
  • userauth request encoding
  • MIC data construction
  • token/MIC packet ordering
  • exchange-complete packet ordering

Design Notes

This PR intentionally does not add a dependency on a system GSSAPI library.

Different callers may want to use platform GSS.framework, MIT Kerberos, Heimdal, SSPI, or a fake/test implementation. The trait boundary keeps that platform policy outside russh.

The intended layering is:

russh:
  SSH RFC 4462 protocol flow
  packet constants
  auth state machine
  token/MIC packet encoding
  `GssapiAuthenticator` trait boundary

caller:
  platform GSSAPI/Kerberos/SSPI implementation
  mechanism OID selection
  service principal policy
  credential/cache behavior

Verification

cargo fmt --check
cargo test -p russh gssapi
cargo check -p russh
cargo test -p russh

[Eugeny]

Thank you! @all-contributors add @ayamir for code

[allcontributors]

@Eugeny

I've put up a pull request to add @ayamir! 🎉