[settings] simplify and enhance Settings (#6683)

This commit updates the `Settings` implementation. It adds template
versions of methods `Read<EntryType>()`, `Save<EntryType>()`, and
`Delete<EntryType>()` which make it easier for other modules to use,
while ensuing to share a common underlying implementation for all
entry types.

The settings entry types (e.g., `NetworkInfo`, `ParentInfo`, etc.) are
changed in this commit to provide a constant `kKey` specifying the
associated setting key with the entry. For simple entries, a related
class is added to define the `kKey` constant and the associated value
type (e.g., `OmrPrefix` defines `Ip6::Prefix` as its `ValueType`).
This model makes it easier to add new setting keys and entries in
future.

This commit also simplifies the logging in `Settings`. The entry types
now provide `Log()` method to log their data and a common
`Settings::Log()` method is added which handles all logging (on
success or in case of an error).

Finally, this commit moves the `SettingsDriver` definition to its own
`common/settings_driver.hpp` header file and and ensures that its
methods are defined as inline.

src/core/thread/dua_manager.cpp

@@ -213,7 +213,7 @@ void DuaManager::Restore(void)
 {
     Settings::DadInfo dadInfo;
 
-    SuccessOrExit(Get<Settings>().ReadDadInfo(dadInfo));
+    SuccessOrExit(Get<Settings>().Read(dadInfo));
     mDadCounter = dadInfo.GetDadCounter();
 
 exit:
@@ -225,7 +225,7 @@ Error DuaManager::Store(void)
     Settings::DadInfo dadInfo;
 
     dadInfo.SetDadCounter(mDadCounter);
-    return Get<Settings>().SaveDadInfo(dadInfo);
+    return Get<Settings>().Save(dadInfo);
 }
 
 void DuaManager::AddDomainUnicastAddress(void)