Add client tests and improve memory handling in tests

[thomvet]

This PR contains the following high level points:

• introduces tests for non-async client read, client write and client service calls.
• remove generation code for UA_Client_readXXXAttribute_async functions from client.jl (these are not inlined, hence contained in open62541.jl directly, also evidenced from empty attributes_UA_Client_read_async variable in generated_defs.jl)
• improve memory handling within testsets in general
• removes auto-generated docstrings from open62541.jl
• complete docstrings within client.jl
• transfer remaining high level functions to highlevel_client.jl
• restructure async functions in client.jl
• added additional high level type JUA_ExpandedNodeId
• added additional high level type JUA_LocalizedText

[codecov]

Codecov Report

Attention: Patch coverage is 80.35714% with 33 lines in your changes missing coverage. Please review.

Project coverage is 43.99%. Comparing base (9a85856) to head (c651ca7).

Files	Patch %	Lines
src/highlevel_types.jl	83.33%	14 Missing ⚠️
src/client.jl	29.41%	12 Missing ⚠️
src/highlevel_client.jl	20.00%	4 Missing ⚠️
src/highlevel_server.jl	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   41.74%   43.99%   +2.24%     
==========================================
  Files          12       13       +1     
  Lines        3799     3923     +124     
==========================================
+ Hits         1586     1726     +140     
+ Misses       2213     2197      -16     

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[martinkosch]

Wow, again an impressive amount of work. So far it looks good to me!

[thomvet]

I will put some more structure into the documentation and then proceed with merging this.
Once that happened, I feel that the package overall would be ready for registration in a version 0.1.0.

We've got to start somewhere and it doesn't have to be perfect from the start.