Allow adding health check callback to Dapr App run in gRPC

[evhen14]

Description

Add health check support to the SDK for gRPC following the example of the go-sdk. The dapr docs state that the /dapr.proto.runtime.v1.AppCallbackHealthCheck/HealthCheck endpoint is called, but this SDK doesn't support specifying that method implementation.

The suggested way to use it would be app.register_health_check(health_check_func)

Use case 1: gRPC endpoints depend on other components to function properly. The health checks callback can be used to validate the liveness of the other components

Use case 2: Run gRPC app next to a web app in the same container in a separate thread. The entire app is healthy only when both dapr app and web app are healthy. I expect that dapr thread could die or stop being responsive, hence the way to add health checks.

Issue reference

It's the outcome of the issue #722 I raised earlier where I see a gap for adding meaningful health checks

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

[passuied]

typo: should be _health_servicer.py

[berndverst]

Please run tox -e ruff for our autoformatter of source files.

[berndverst]

Great contribution - and LGTM to me. Please run tox -e ruff to apply our autoformatter for code and commit the changes.

I'll approve and merge once that is done.

[yevgen-el8]

@berndverst tox -e ruff was applied. Thx for your feedback!

[berndverst]

ext/dapr-ext-grpc/dapr/ext/grpc/_health_servicer.py:17: error: Function "cb" could always be true in boolean context  [truthy-function]
ext/dapr-ext-grpc/dapr/ext/grpc/_health_servicer.py:22: error: Attribute "_health_check_cb" already defined on line 19  [no-redef]
ext/dapr-ext-grpc/dapr/ext/grpc/_health_servicer.py:22: error: Incompatible types in assignment (expression has type "None", variable has type "Callable[[], None]")  [assignment]

[codecov]

Codecov Report

Attention: Patch coverage is 97.26027% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.22%. Comparing base (fc0e9d1) to head (eed11ed).
Report is 25 commits behind head on main.

Files	Patch %	Lines
...xt/dapr-ext-grpc/dapr/ext/grpc/_health_servicer.py	94.73%	1 Missing ⚠️
ext/dapr-ext-grpc/tests/test_app.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #723      +/-   ##
==========================================
- Coverage   86.37%   86.22%   -0.15%     
==========================================
  Files          79       79              
  Lines        4094     4166      +72     
==========================================
+ Hits         3536     3592      +56     
- Misses        558      574      +16     

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[marcduiker]

@holopin-bot @evhen14 Thank you!

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