[shim] Log successful API calls with trace level

[un-def]

Reduces log noise when DSTACK_SHIM_LOG_LEVEL is set to 5 (debug) or lower

Such messages are gone:

Oct 29 13:41:15 defpc dstack-shim[389812]: time=2025-10-29T13:41:15.1726Z level=debug endpoint=/api/healthcheck status=200 method=GET
Oct 29 13:41:15 defpc dstack-shim[389812]: time=2025-10-29T13:41:15.174582Z level=debug method=GET endpoint=/api/tasks status=200
Oct 29 13:41:29 defpc dstack-shim[389812]: time=2025-10-29T13:41:29.307652Z level=debug method=GET endpoint=/api/healthcheck status=200
Oct 29 13:41:29 defpc dstack-shim[389812]: time=2025-10-29T13:41:29.309535Z level=debug method=GET endpoint=/api/tasks status=200
Oct 29 13:41:40 defpc dstack-shim[389812]: time=2025-10-29T13:41:40.535613Z level=debug method=GET endpoint=/api/healthcheck status=200