Create an API endpoint and associated test that, given the name of a traffic split, returns the following information about the apex service of the traffic split, and its leaf services.

In this example, the name of the traffic split is foo-ts and the root service is foo.

Note: this ticket is for the API endpoint only, not CLI/Dashboard.

NAME     APEX    LEAF      WEIGHT     SUCCESS    RPS      LATENCY_P50   LATENCY_P95   LATENCY_P99
foo-ts   foo     foo-v1    90m        100.00%    9.0rps   1ms           1ms           1ms
foo-ts   foo     foo-v2    10m        91.25%     1.0rps   3ms           4ms           4ms