[WIP] Provide callback for AccessLevel attribute

[matkonnerth]

In some situations the write access to a variable node is not possible, e.g. the backend is not connected or device/machine is in a state where change of a certain variable is not possible. For such use cases a callback could be provided, which calculates the accessLevel.

For me the user access level attribute should consider only the user/role permissions attached to the session.

Todos:
add tests
same callback should be possible for the executable flag for methods -> executable flag is a boolean
Move accessLevel and Callback in an union type -> doesn't really make sense, if callback is there, invoke it

[jpfr]

This is a good feature.
Please indicate the relation of this feature with getUserAccessLevel in the AccessControl plugin.
Furthermore, there are probably many other cases where we want to override the nodestore.
I would prefer a solution that works for all attributes and not only for AccessLevel.
For example with a bitmask that indicates which attribute is "overridden".

open62541/include/open62541/plugin/accesscontrol.h

Line 60 in 8f4f99d

UA_Byte (*getUserAccessLevel)(UA_Server *server, UA_AccessControl *ac,

[matkonnerth]

Makes sense for me, I've changed the callback for reading a generic attribute value. I haven't considered writing an attribute up to now. If this is really necessary, the callback for reading/writing attributes should look like the DataSource callback.

[matkonnerth]

have to think of the writeMask and using DataSource struct for attribute read/write

[matkonnerth]

switch back to WIP, this is only experimental