cleanup(rest): only trace if enabled

[dbolduc]

We were always including the REST tracing client. Now, only include it if tracing is enabled, as determined by the Options.

Make(Default|Pooled)RestClient(...) are one of those interfaces (like Make*Connection()) that are difficult to cover with unit tests. If someone asks, I can try to cover it in an integration test.

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NOTE: I also changed a

std::make_shared<DefaultCurlHandleFactory>(options);

to a

GetDefaultCurlHandleFactory(options);

The difference is that GetDefaultCurlHandleFactory(options) checks if CARootsFilePathOption is empty, before using it and CAPathOption:

google-cloud-cpp/google/cloud/internal/curl_handle_factory.cc

Line 39 in 08e52b4

if (!options.get<CARootsFilePathOption>().empty()) {

I think that is the one we want. @coryan can you speak to this?

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This change is 

[codecov]

Codecov Report

Patch coverage: 92.30% and project coverage change: -0.01 ⚠️

Comparison is base (08e52b4) 93.68% compared to head (cfb9cfa) 93.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12098      +/-   ##
==========================================
- Coverage   93.68%   93.68%   -0.01%     
==========================================
  Files        2012     2012              
  Lines      175713   175716       +3     
==========================================
- Hits       164619   164616       -3     
- Misses      11094    11100       +6     

Impacted Files	Coverage Δ
google/cloud/internal/curl_rest_client.cc	98.57% <92.30%> (+2.22%)	⬆️

... and 4 files with indirect coverage changes

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