add direct channel feature

[akaszynski]

Resolve #987 by allowing for instantiating MapdlGrpc directly from a gRPC channel.

Behind the scenes, it's still running a multi-connect and reuses much of the existing "hardware".

If you wish to customize the channel, you can also directly connect
directly to gRPC channels. For example, if you wanted to create an insecure
channel with a maximum message length of 8 MB.

>>> import grpc
>>> channel = grpc.insecure_channel(
...     '127.0.0.1:50052',
...     options=[
...         ("grpc.max_receive_message_length", 8*1024**2),
...     ],
... )
>>> mapdl = pymapdl.Mapdl(channel=channel)

[codecov]

Codecov Report

Merging #996 (c45e2e2) into main (bd80c49) will increase coverage by 1.50%.
The diff coverage is 97.91%.

@@            Coverage Diff             @@
##             main     #996      +/-   ##
==========================================
+ Coverage   73.28%   74.78%   +1.50%     
==========================================
  Files          39       43       +4     
  Lines        5750     6037     +287     
==========================================
+ Hits         4214     4515     +301     
+ Misses       1536     1522      -14     

[plule-ansys]

This looks good, thank you Alex