Alternative grpcbox_channel stop function

[psalin]

Adds stop/2 for providing more graceful termination of a channel.
stop/1 will kill h2 streams which causes crash reports. For a similar
termination without crash reports grpcbox_channel:stop(Name, shutdown)
can now be called. Calling grpcbox_channel:stop(Name, normal) will
shutdown the h2_connection with stop instead of calling exit for it.

This addresses the issue described in #58

[codecov]

Codecov Report

Base: 39.16% // Head: 39.43% // Increases project coverage by +0.27% 🎉

Coverage data is based on head (d2d7813) compared to base (5ea123f).
Patch coverage: 60.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
+ Coverage   39.16%   39.43%   +0.27%     
==========================================
  Files          28       28              
  Lines        2099     2107       +8     
==========================================
+ Hits          822      831       +9     
+ Misses       1277     1276       -1     

Impacted Files	Coverage Δ
src/grpcbox_channel.erl	66.66% <40.00%> (-2.09%)	⬇️
src/grpcbox_subchannel.erl	70.00% <80.00%> (+12.85%)	⬆️

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[psalin]

I think this one actually might cause a report for the original way of stopping now that was not there before. Will try out it out a bit more and change it so that the original behavior is the same.

[psalin]

Fixed conflicts with the base branch. This one now works so that the original way of stopping with stop/1 works as before, while stop/2 provides a way to choose a more graceful shutdown.