There are some fields that exist in Netplan but not in the system and
vice-versa, such as on-link and protocol. Using the default dataclass
methods makes it harder to compare objects.

The state_diff submodule is responsible for comparing system's and
netplan's states and calculating the differences.

The new class NetplanDiffState must be instantiated with both states.
The method get_full_state() will return both system's and netplan's
states without calculating the differences.

The diff calculation will be added in the next commits.

The get_diff() will calculate the differences between system's and
Netplan's states and return a report to the caller.

This commit adds support for checking what interfaces are present only
in one of the states.

get_diff() will compare and report differences in the addresses present
in the system and Netplan. Addresses detected as dynamically assigned
will not be reported as missing as they will not be defined in Netplan.

A number of heuristics are implemented to eliminate routes we don't want
to compare, such as routes installed dynamically, as they will not be
found in Netplan.

It will be used to serialize the state and diff.

For netplan interfaces add the name and type. For system interfaces add
the name, type and index.

Also add a few integrations tests that use tools/diff.py to test the
diff against a running system configured with Netplan.