Turn hs_subcredential_t into a proper struct.

src/core/crypto/hs_ntor.c

src/core/crypto/hs_ntor.h

src/feature/hs/hs_cell.c

src/feature/hs/hs_cell.h

src/feature/hs/hs_circuit.c

src/feature/hs/hs_circuit.h

src/feature/hs/hs_client.c

src/feature/hs/hs_common.c

src/feature/hs/hs_common.h

src/feature/hs/hs_descriptor.c

src/feature/hs/hs_descriptor.h

src/feature/hs/hs_ob.c

src/feature/hs/hs_ob.h

src/feature/hs/hs_service.c

src/test/fuzz/fuzz_hsdescv3.c

src/test/hs_test_helpers.c

src/test/hs_test_helpers.h

src/test/test_hs_cache.c

src/test/test_hs_client.c

src/test/test_hs_descriptor.c