Create super-class OperationalDatasetView out of OperationalDataset (project-chip#40346)

* Create super-class OperationalDatasetView out of OperationalDataset

OperationalDatasetView does not copy or hold a buffer for the underlying
dataset, and provides all the read-only methods that were present on
OperationalDataset.  OperationalDataset has its own buffer and copies the
dataset as before.

Also simplify some of the mechanics of updating values in the dataset, by
replacing MakeRoom with InsertOrReplace that directly takes the required size
of the value and handles updating both the tag and length of the TLV item as
well as of the overall dataset, so the caller only has to fill in the value.

* Avoid out of bounds access in IsValid

* use EXPECT_STREQ instead of strncmp in test

* Handle copy construction / assignment correctly

* Fix test warning and use the view in TND server

* Address review comments

- Replace ThreadTLV::kMaxLength with kMaxDatasetElementLength outside of it,
  the TheadTLV helper does not need to know about kSizeOperationalDataset.
- assert(aValueSize <= kMaxDatasetElementLength) in InsertOrReplace and
  document that callers (the various Set... methods) are responsible for
  checking the value size.
- Check Channel Page == 0 in GetChannel and use BigEndian instead of math.
- Tidy up method descriptions in the header.

* Expand comment on self-assignments

Author: ksperling-apple

src/app/clusters/thread-network-directory-server/thread-network-directory-server.cpp

@@ -180,7 +180,7 @@ void ThreadNetworkDirectoryServer::InvokeCommand(HandlerContext & ctx)
 void ThreadNetworkDirectoryServer::HandleAddNetworkRequest(HandlerContext & ctx,
                                                            const ThreadNetworkDirectory::Commands::AddNetwork::DecodableType & req)
 {
-    OperationalDataset dataset;
+    OperationalDatasetView dataset;
     ByteSpan extendedPanIdSpan;
     uint64_t activeTimestamp;
     union
@@ -200,7 +200,6 @@ void ThreadNetworkDirectoryServer::HandleAddNetworkRequest(HandlerContext & ctx,
     CHIP_ERROR err;
     auto status          = IMStatus::ConstraintError;
     const char * context = nullptr;
-    // TODO: An immutable OperationalDatasetView on top of a ByteSpan (without copying) would be useful here.
     SuccessOrExitAction(err = dataset.Init(req.operationalDataset), context = "OperationalDataset");
     SuccessOrExitAction(err = dataset.GetExtendedPanIdAsByteSpan(extendedPanIdSpan), context = "ExtendedPanID");
     SuccessOrExitAction(err = dataset.GetActiveTimestamp(activeTimestamp), context = "ActiveTimestamp");