feat(pubsub): implement GUAC for SchemaAdmin

[coryan]

Part of the work for #6463

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[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #7436 (d6ad1b6) into main (1e29dce) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #7436      +/-   ##
==========================================
+ Coverage   93.63%   93.64%   +0.01%     
==========================================
  Files        1359     1362       +3     
  Lines      118082   118232     +150     
==========================================
+ Hits       110568   110721     +153     
+ Misses       7514     7511       -3     

Impacted Files	Coverage Δ
google/cloud/pubsub/internal/schema_stub.h	100.00% <ø> (ø)
...integration_tests/schema_admin_integration_test.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/internal/schema_auth.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/internal/schema_auth.h	100.00% <100.00%> (ø)
google/cloud/pubsub/internal/schema_auth_test.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/internal/schema_stub.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/schema_admin_connection.cc	91.91% <100.00%> (+1.79%)	⬆️
google/cloud/pubsub/subscriber_connection_test.cc	97.18% <0.00%> (-0.71%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.62% <0.00%> (+0.37%)	⬆️
... and 2 more

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[dbolduc]

(Sorry this comment comes a little late (it applies to previous PR's). Also, feel free to ignore it. I'm just throwing an idea out there.)

There is something weird about this method, which is only used for testing. It is used in conjunction with:

google-cloud-cpp/google/cloud/pubsub/schema_admin_connection_test.cc

Lines 42 to 48 in d6ad1b6

	std::shared_ptr<SchemaAdminConnection> MakeTestSchemaAdminConnection(
	std::shared_ptr<pubsub_internal::SchemaStub> mock, Options opts = {}) {
	opts = pubsub_internal::DefaultCommonOptions(
	pubsub_testing::MakeTestOptions(std::move(opts)));
	return pubsub_internal::MakeSchemaAdminConnection(std::move(opts),
	std::move(mock));
	}

We need it in order to make the SchemaAdminConnectionImpl, which is defined in an anonymous namespace. Part of me wants to combine the two functions, but we probably stay more flexible by leaving them separate.

I did get confused just now about where I was in the code. We could name it something like MakeTestSchemaAdminConnection and s/stub/mock/. Or potentially add a comment that says it is only used for testing, and that is why we use the insecure credentials.

Also, I am wondering if we should even be setting the unified credentials here. I actually think it makes more sense to set them in schema_admin_connection_test.cc. The thing that is testing decides that we are using test credentials. What do you think?

[coryan]

See #7438, let's continue the discussion there.

[dbolduc]

I am approving. If we think there is value in changing the test-only Make*Connection methods, we can do so in a future PR that updates them all at once.