[dataset] add ValidateTlvs(), helper methods, and unit test (#10111)

This commit enhances dataset handling:

- Adds `ValidateTlvs()` which parses and validates all known TLVs
  within the `Dataset` and checks for any duplicates.
- Introduces a set of `SetFrom()` methods for constructing a `Dataset`
  from various types of inputs, e.g., other `Dataset`, TLV sequences,
  `DatasetInfo` structures, bytes read from `Message`. Now they
  consistently clear the `Dataset` before setting it.
- Adds `WriteTlvsFrom()` to update a `Dataset` replacing/appending a
  set of TLVs.
- Adds `AppendTlvsFrom()` to append an already encoded sequence of
  TLVs without validating/checking the format.
- Renames `Get/SetSize()` to `Get/SetLength()` for consistency.
- Adds `test_dataset.cpp` unit test for basic `Dataset` validation.

src/cli/cli_dataset.cpp

@@ -612,7 +612,7 @@ template <> otError Dataset::Process<Cmd("init")>(Arg aArgs[])
         otOperationalDataset dataset;
 
         SuccessOrExit(error = otDatasetCreateNewNetwork(GetInstancePtr(), &dataset));
-        SuccessOrExit(error = otDatasetConvertToTlvs(&dataset, &sDatasetTlvs));
+        otDatasetConvertToTlvs(&dataset, &sDatasetTlvs);
     }
 #endif
     else if (aArgs[0] == "tlvs")
@@ -1105,7 +1105,7 @@ template <> otError Dataset::Process<Cmd("set")>(Arg aArgs[])
     {
         otOperationalDataset     dataset;
         otOperationalDatasetTlvs datasetTlvs;
-        uint16_t                 tlvsLength = MeshCoP::Dataset::kMaxSize;
+        uint16_t                 tlvsLength = OT_OPERATIONAL_DATASET_MAX_LENGTH;
 
         SuccessOrExit(error = aArgs[1].ParseAsHexString(tlvsLength, datasetTlvs.mTlvs));
         datasetTlvs.mLength = static_cast<uint8_t>(tlvsLength);