API/TL/UCP: Add per-team endpoint traffic class (QoS)

[ybenvidia]

What

Allow a UCC team to be created with an endpoint traffic class so all UCX
endpoints opened for that team use a chosen DSCP/priority.

Why

Enables per-team (≈ per communicator / process-group) QoS: run a
higher-priority collective on a team tagged with a higher-priority traffic
class, and let the NIC/fabric arbitrate between flows accordingly.

Changes

• Public API: new ucc_team_params.ep_traffic_class field, enabled with
  UCC_TEAM_PARAM_FIELD_EP_TRAFFIC_CLASS.
• Plumbing: ucc_team_create_post → tl_ucp team
  (UCC_CLASS_INIT_FUNC(ucc_tl_ucp_team_t)) → ucc_tl_ucp_connect_ep, which
  sets ucp_ep_params.ep_traffic_class + UCP_EP_PARAM_FIELD_EP_TRAFFIC_CLASS.
• Sentinel UCC_TL_UCP_NO_TCLASS for "not set"; service/internal teams keep
  the default.
• ucc_perftest: new UCC_PT_EP_TRAFFIC_CLASS env to set the team's traffic
  class for benchmarking.

Dependency

Requires a UCX with the per-endpoint traffic class API
(UCP_EP_PARAM_FIELD_EP_TRAFFIC_CLASS / ucp_ep_params.ep_traffic_class) —
see the companion UCX PR.

Compatibility

Opt-in: teams created without the field mask bit behave exactly as before.

Testing

• ucc_perftest -c allreduce with UCC_PT_EP_TRAFFIC_CLASS=<dscp> on 2 nodes:
  the tagged team's endpoints open QPs with the configured DSCP, verified down
  to the RoCEv2 packets on the wire.