Advertise transport selected in user-agent string

src/core/lib/channel/channel_stack.c

src/core/lib/channel/channel_stack.h

src/core/lib/channel/channel_stack_builder.c

src/core/lib/channel/http_client_filter.c

src/core/lib/channel/http_client_filter.h

test/core/channel/channel_stack_test.c