[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.

tests/unit/test_srp_server.cpp

@@ -205,7 +205,7 @@ void InitTest(void)
     otOperationalDatasetTlvs datasetTlvs;
 
     SuccessOrQuit(otDatasetCreateNewNetwork(sInstance, &dataset));
-    SuccessOrQuit(otDatasetConvertToTlvs(&dataset, &datasetTlvs));
+    otDatasetConvertToTlvs(&dataset, &datasetTlvs);
     SuccessOrQuit(otDatasetSetActiveTlvs(sInstance, &datasetTlvs));
 
     SuccessOrQuit(otIp6SetEnabled(sInstance, true));