transport/client: Return status code Unknown on malformed grpc-status

[eshitachandwani]

Fixes : #8713

Unknown is defined as follows in grpc/grpc@master/doc/statuscodes.md

Unknown error. For example, this error may be returned when a Status value received 
from another address space belongs to an error space that is not known in this 
address space. Also errors raised by APIs that do not return enough error information
may be converted to this error.

It also mentions of returning Unknown for parsing errors in the table in the above doc.

We are currently returning Internal for status parsing errors as well, which is contrary to what is mentioned in the above spec. This PR changes it to return Unknown.

RELEASE NOTES:

• transport/client : Return status code Unknown on malformed grpc-status.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.37%. Comparing base (432bda3) to head (4114e93).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8735      +/-   ##
==========================================
+ Coverage   83.22%   83.37%   +0.15%     
==========================================
  Files         419      419              
  Lines       32454    32454              
==========================================
+ Hits        27009    27060      +51     
+ Misses       4057     4028      -29     
+ Partials     1388     1366      -22     

Files with missing lines	Coverage Δ
internal/transport/http2_client.go	92.51% <100.00%> (-0.10%)	⬇️

... and 25 files with indirect coverage changes

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