[tlv] improve encapsulation and docs for Tlv::Info (#12301)

This change renames `Tlv::ParsedInfo` to `Tlv::Info` to make it more
concise and to better reflect its purpose as a metadata holder for a
TLV in a message.

The member variables of `Tlv::Info` are made private and public
accessor methods are introduced to interact with the object's
contenet. This helps ensure that the internal representation of the
parsed TLV information is not modified directly by external modules.
All existing caller are updated to use the new name and the public
helper getter methods.

The Doxygen comments of `Tlv::Info` are also improved for better
clarity. Unit test `test_tlv` is also updated to validate all the
new methods.

src/core/thread/mle_ftd.cpp

@@ -2707,7 +2707,7 @@ void Mle::HandleDiscoveryRequest(RxInfo &aRxInfo)
 {
     Error                             error                     = kErrorNone;
     bool                              parsedDiscoveryRequestTlv = false;
-    Tlv::ParsedInfo                   tlvInfo;
+    Tlv::Info                         tlvInfo;
     MeshCoP::DiscoveryRequestTlvValue discoveryRequestTlvValue;
     MeshCoP::ExtendedPanId            extPanId;
     OffsetRange                       offsetRange;
@@ -2723,12 +2723,12 @@ void Mle::HandleDiscoveryRequest(RxInfo &aRxInfo)
     {
         SuccessOrExit(error = tlvInfo.ParseFrom(aRxInfo.mMessage, offsetRange));
 
-        if (tlvInfo.mIsExtended)
+        if (tlvInfo.IsExtended())
         {
             continue;
         }
 
-        switch (tlvInfo.mType)
+        switch (tlvInfo.GetType())
         {
         case MeshCoP::Tlv::kDiscoveryRequest:
             SuccessOrExit(error = Tlv::Read<MeshCoP::DiscoveryRequestTlv>(aRxInfo.mMessage, offsetRange.GetOffset(),