fix(pubsub): export two more functions

[RolfKal]

This will allow compilation of examples/pubsub/server_pubsub_file_configuration.c when compiling open62541 library as shared library under Windows

[jpfr]

I would split this as follows:

• Add a method for parsing the PubSubConfiguration from the binary file (as a ByteString)
• Add a method for loading the (already parsed) PubSubConfiguration(2).

That way, we are flexible to add JSON as a configuration format in the future.

As all the parts are already implemented, it is just a question of separating the concerns.