* Reduction of network access

Cache the nodes of potentially multiple-used FileType methods.

* indentation error corrected

* Add low-level file system functionality

* Added examples for the use of ua_file_transfer.py

In the following you will find examples for the use of the
classes UaFile and UaDirectory and how to handle typical uaerrors.
see: ./asyncua/client/ua_file_transfer.py.

The OPC UA File Transfer specification can be found here:
https://reference.opcfoundation.org/Core/docs/Part5/C.1/

* pylint optimized - ua_file_transfer.py 

pylint optimization of file ua_file_transfer.py.
=> Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

* pylint optimized - client_ua_file_transfer.py

pylint optimization of client_ua_file_transfer.py.
=> Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)
Co-authored-by: Curious Crook <privat@nowhere.org>

Augmented assignment statements became available with Python 2.
https://docs.python.org/3/whatsnew/2.0.html#augmented-assignment

Thus improve three source code places accordingly.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>

The test suite used to create files in the CWD, which then showed up as untracked files in git, and would cause the tests to fail when ran successively with different users.

This commit makes successive test runs independant from each other

* 3x performance improvement for serialization

Instead of re-inspecting the type metadata everytime an object is serialized,
create a single serialization function for each type and just call it when an instance is serialized

* pre-compute uatype array serializers

* Make VariantTypeCustom instances valid cache keys

VariantTypeCustom instances can be used as cache keys for serialization functions

* Enable the server to listen to an arbitrary IP

* Add missing doc
Co-authored-by: oroulet <oroulet@users.noreply.github.com>

With this change you can authenticate via user certificate over sync api.
The application-uri is required, because some servers will reject the connection if the application-uri of the application does not match the one from the client application certificate.

Allow using OptionSets from Addresspace und dynamic creation of OptionSets

Running uals recursively with -l option gives RuntimeWarning:

```
$ uals -d 3 -l
Browsing node i=84 at opc.tcp://localhost:4840

DisplayName                    NodeId                    BrowseName                DataType   Timestamp                      Value

Traceback (most recent call last):
  File ".../opcua-asyncio/tools/uals", line 11, in <module>
    uals()
  File ".../opcua-asyncio/asyncua/tools.py", line 342, in uals
    asyncio.run(_uals())
  File "/usr/lib/python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/usr/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File ".../opcua-asyncio/asyncua/tools.py", line 368, in _uals
    await _lsprint_long(node, args.depth - 1)
  File ".../opcua-asyncio/asyncua/tools.py", line 442, in _lsprint_long
    name, bname, nclass, mask, umask, dtype, val = [attr.Value.Value for attr in attrs]
TypeError: 'coroutine' object is not iterable
sys:1: RuntimeWarning: coroutine 'Node.read_attributes' was never awaited
```

Support OptionSets from Part5 7.17 like UadpNetworkMessageContentMask. In nodesets they are UAEnumerations with attribut "IsOptionSet" = True

Fixed generation for default values for GUID. Now an empty GUID is generated.

When a client reconnects, it can require to clear its security_policy
settings to deal with situations where the peer_certificate has
changed for example (regular server cert renewal).

This PR introduces the `reset_security_policy()` client method to handle
this use-case, and adds it to the HaClient reconnection mechanism.

Note: We could arguably invoke this method from `set_security()` as they
often will be called together, but according to [OPC-UA Part4 5.4.4](https://reference.opcfoundation.org/v104/Core/docs/Part4/5.4.4/),
the GetEndpoints service may require TLS and so enforcing an empty policy would break this use-case.

The OPC UA specification does not forbid that a server does not offer DiscoveryUrls. This can lead to a crash when calling the function application_to_strings().

Check DiscoveryUrls for None or empty before iterating to avoid the crash described in #1127.

* extended ServerItemCallback class with 'event_comes_from_internal_session' parameter to distinguish events triggered from server to the ones triggered from OPC-UA clients

* Fixed arguments order

* Renamed variable to is_external
Co-authored-by: Marco Panato <marco.panato_01@univr.it>