feat: support `functions::Function` in `Run()`

[coryan]

Application developers can use functions::Function as a wrapper for their functions in unit and integration tests. Note that we need to change the buildpacks (in a separate repository) to completely support this feature.

[codecov[bot]]

Codecov Report

Merging #340 (ca48f3b) into main (e95a5d8) will decrease coverage by 0.04%. The diff coverage is 89.65%.

@@            Coverage Diff             @@
##             main     #340      +/-   ##
==========================================
- Coverage   89.29%   89.24%   -0.05%     
==========================================
  Files          98       98              
  Lines        2727     2733       +6     
==========================================
+ Hits         2435     2439       +4     
- Misses        292      294       +2     

Impacted Files	Coverage Δ
google/cloud/functions/internal/framework_impl.cc	72.85% <82.35%> (-1.39%)	:arrow_down:
...tions/integration_tests/cloud_event_conformance.cc	95.83% <100.00%> (ø)
...functions/integration_tests/cloud_event_handler.cc	100.00% <100.00%> (ø)
...e/cloud/functions/integration_tests/echo_server.cc	100.00% <100.00%> (ø)
...ud/functions/integration_tests/http_conformance.cc	100.00% <100.00%> (ø)
...le/cloud/functions/internal/framework_impl_test.cc	95.34% <100.00%> (+0.11%)	:arrow_up:

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[coryan]

You tripped the auto merge, I will send those fixes in a separate PR.