[coap] misc enhancements (#5482)

This commit contains a group of smaller changes/renames in `Coap`
modules:
- Add enumeration (c++ style) constants for CoAP `Type` and `Code`
  and `OptionType`.
- Add helpers `Message::Is{Method}Request()` e.g., `IsPostRequest()`
- Define Thread URI Paths (in `uri_paths.hpp/cpp`) as char array.
- Add helper method in `Coap::Message` for common initializations
  e.g., `InitAsConfirmablePost()`.
- Add helper methods for common `Coap:Message` type and code checks
  e.g., `IsConfirmablePost()`.
- Re-define `OT_COAP_TYPE` values to use 2-bit unsigned int values
  (per RFC-7252).

include/openthread/coap.h

@@ -58,6 +58,8 @@ extern "C" {
 
 #define OT_DEFAULT_COAP_PORT 5683 ///< Default CoAP port, as specified in RFC 7252
 
+#define OT_COAP_DEFAULT_TOKEN_LENGTH 2 ///< Default token length.
+
 #define OT_COAP_MAX_TOKEN_LENGTH 8 ///< Max token length as specified (RFC 7252).
 
 #define OT_COAP_MAX_RETRANSMIT 20 ///< Max retransmit supported by OpenThread.
@@ -65,15 +67,15 @@ extern "C" {
 #define OT_COAP_MIN_ACK_TIMEOUT 1000 ///< Minimal ACK timeout in milliseconds supported by OpenThread.
 
 /**
- * CoAP Type values.
+ * CoAP Type values (2 bit unsigned integer).
  *
  */
 typedef enum otCoapType
 {
-    OT_COAP_TYPE_CONFIRMABLE     = 0x00, ///< Confirmable
-    OT_COAP_TYPE_NON_CONFIRMABLE = 0x10, ///< Non-confirmable
-    OT_COAP_TYPE_ACKNOWLEDGMENT  = 0x20, ///< Acknowledgment
-    OT_COAP_TYPE_RESET           = 0x30, ///< Reset
+    OT_COAP_TYPE_CONFIRMABLE     = 0, ///< Confirmable
+    OT_COAP_TYPE_NON_CONFIRMABLE = 1, ///< Non-confirmable
+    OT_COAP_TYPE_ACKNOWLEDGMENT  = 2, ///< Acknowledgment
+    OT_COAP_TYPE_RESET           = 3, ///< Reset
 } otCoapType;
 
 /**

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (26)
+#define OPENTHREAD_API_VERSION (27)
 
 /**
  * @addtogroup api-instance