nanocoap: add payload helper functions

This adds two functions `coap_payload_add()` and `coap_payload_advance()`.

 - `coap_payload_add()` will add n bytes to the payload buffer and advance
    payload pointer accordingly.

    const char hello[] = "Hello CoAP!";
    coap_payload_add(pkt, hello, sizeof(hello));

 - `coap_payload_advance()` will advance the payload buffer after data
    has been added to it.

    int len = snprintf(pkt->payload, pkt->payload_len, "%s %s!", "Hello", "CoAP");
    coap_payload_advance(pkt, len);

I considered adding an additional parameter to keep track of the total request size
(returned size from coap_opt_finish() incremented by each added payload fragment),
but decided against it to keep consistency with the existing API.

sys/include/net/nanocoap.h

@@ -1469,6 +1469,48 @@ int coap_parse(coap_pkt_t *pkt, uint8_t *buf, size_t len);
  */
 void coap_pkt_init(coap_pkt_t *pkt, uint8_t *buf, size_t len, size_t header_len);
 
+/**
+ * @brief   Advance the payload pointer.
+ *
+ * @pre     You added @p len bytes of data to `pkt->payload`.
+ *
+ * You can add payload to a CoAP request by writing data directly to
+ * `pkt->payload`.
+ * This convenience function takes care of advancing the payload pointer
+ * afterwards.
+ *
+ * @param[out]   pkt        pkt to which payload was added
+ * @param[in]    len        length of payload
+ */
+static inline void coap_payload_advance_bytes(coap_pkt_t *pkt, size_t len)
+{
+    pkt->payload += len;
+    pkt->payload_len -= len;
+}
+
+/**
+ * @brief   Add payload data to the CoAP request.
+ *
+ * @pre     @ref coap_opt_finish must have been called before with
+ *               the @ref COAP_OPT_FINISH_PAYLOAD option.
+ *
+ * The function copies @p data into the payload buffer of @p pkt and
+ * advances the payload pointer.
+ *
+ * This is just a convenience function, you can also directly write
+ * to `pkt->payload` if you have a function that outputs payload to
+ * a buffer.
+ * In this case you should instead call @ref coap_payload_advance_bytes.
+ *
+ * @param[out]   pkt        pkt to add payload to
+ * @param[in]    data       payload data
+ * @param[in]    len        length of payload
+ *
+ * @returns      number of payload bytes added on success
+ * @returns      < 0 on error
+ */
+ssize_t coap_payload_put_bytes(coap_pkt_t *pkt, const void *data, size_t len);
+
 /**
  * @brief   Create CoAP reply (convenience function)
  *

sys/net/application_layer/nanocoap/nanocoap.c

@@ -922,6 +922,18 @@ ssize_t coap_opt_finish(coap_pkt_t *pkt, uint16_t flags)
     return pkt->payload - (uint8_t *)pkt->hdr;
 }
 
+ssize_t coap_payload_put_bytes(coap_pkt_t *pkt, const void *data, size_t len)
+{
+    if (pkt->payload_len < len) {
+        return -ENOSPC;
+    }
+
+    memcpy(pkt->payload, data, len);
+    coap_payload_advance_bytes(pkt, len);
+
+    return len;
+}
+
 void coap_block_object_init(coap_block1_t *block, size_t blknum, size_t blksize,
                             int more)
 {