This commit adds `Dns::Name` type which provides a set of helper
methods to encode/decode DNS names. `AppendName()` method encodes and
appends a full name (e.g., "test.example.com") to a message. Other
helper methods enable appending labels (or groups of labels) and/or
constructing a compressed name (using pointer labels). `ParseName()`
method parses and skips over a full name in a message. `ReadLabel()`
method reads labels one by one and works independently of whether the
encoded name is compressed or not. `ReadName()` method read an entire
name from message. The new methods are used in `Dns::Client`.
This commit also add a unit test `test-dns` covering the behavior of
the newly added helper methods.
Finally, this commit adds few helper methods in `ResourceRecords` to
get the size and check the type of a DNS record.
This commit adds a new template class `ot::String<size>` which
implements a fixed-length character string. This class is then used as
return value of `ToString()` methods from different classes. This
simplifies the `ToString()` implementation and its use. This commit
also adds a unit test for `String`.
Abtin Keshavarzian