Add Temporal workflow

[mincong-h]

Overview

Add samples about how to write workflows in Temporal and how to test them.

Testing

The new tests are taken into account in CI (https://github.com/mincong-h/learning-go/runs/3850524314):

=== RUN   TestWorkflowTestSuite
=== RUN   TestWorkflowTestSuite/TestWorkflow
2021/10/10 08:28:10 DEBUG handleActivityResult: *workflowservice.RespondActivityTaskCompletedRequest. ActivityID 1 ActivityType MyActivity
2021/10/10 08:28:10 INFO  Workflow completed. result Hello, UnitTest!
--- PASS: TestWorkflowTestSuite (0.00s)
    --- PASS: TestWorkflowTestSuite/TestWorkflow (0.00s)
PASS
ok  	github.com/mincong-h/learning-go/temporal/error_retries	0.008s

Blog

https://mincong.io/en/error-retries-in-temporal/