1. in tos_at.h, added int fuzzy_matching; field into at_echo_st struct, if this field is set to K_TRUE, then if echo message contains the string in "echo_expect" field.
2. added __API__ int tos_at_echo_fuzzy_matching_create(at_echo_t *echo, char *buffer, size_t buffer_size, char *echo_expect_contains) api to tos_at.c, which will create an at_echo_t with fuzzy_matching = K_TRUE;
3. added RHF76_ATCMD_SET_DELAY and rhf76_set_delay to RHF76.h to allow set/query RX delay config
4. added RHF76_ATCMD_SET_DATA_RATE and rhf76_set_data_rate to RHF76.h to allow set/query date rate config
5. added rhf76_at_cmd_exe for DEBUG purpose, so that user can execute any AT+ commands they want
6. added code in lora_demo.c to demonstrate package segmentation.
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