add TraceID and TraceParent handling for grpc and client

[majkio]

Description

Added support for extracting TraceId and TraceParent from Cloud Events Extension

Issue reference

Please reference the issue this PR will close: #355

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation

[mikeee]

I think it'd be good to consolidate the getStringValueFromExtension() helper function to an internal package e.g. internal/structpb so that we're not duplicating code unnecessarily. I'd also in an ideal world like to see some debugging to differentiate between whether a key has a value declared as "" or whether the key is missing alltogether. What do you think?