gRPC async invoke

[olegbespalov]

What?

This PR refactors the logic of the grpc's unary Invoke so that it can be reused in the AsyncInvoke and also implements AsyncInvoke itself.

It's better to check the changes by the commits.

Why?

At the k6-core, we agreed that the modern way of defining API is the async one.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

Closes #3550

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.37209% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 73.34%. Comparing base (144209f) to head (7f040a8).

❗ Current head 7f040a8 differs from pull request most recent head 95fff69. Consider uploading reports for the commit 95fff69 to get more accurate results

Files	Patch %	Lines
js/modules/k6/grpc/client.go	85.29%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3693      +/-   ##
==========================================
+ Coverage   73.32%   73.34%   +0.02%     
==========================================
  Files         278      278              
  Lines       20332    20358      +26     
==========================================
+ Hits        14908    14932      +24     
- Misses       4480     4481       +1     
- Partials      944      945       +1     

Flag	Coverage Δ
macos	73.25% <88.37%> (+0.01%)	⬆️
ubuntu	73.27% <88.37%> (+0.01%)	⬆️
windows	73.19% <88.37%> (+0.01%)	⬆️

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