feat: add schema service to gapic

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PiperOrigin-RevId: 348038736
Source-Link: googleapis/googleapis@41d8fbf
PiperOrigin-RevId: 336174031
Source-Link: googleapis/googleapis@bf79839

[codecov]

Codecov Report

Merging #494 (6e55532) into master (4123f26) will decrease coverage by 0.35%.
The diff coverage is 80.10%.

@@             Coverage Diff              @@
##             master     #494      +/-   ##
============================================
- Coverage     79.14%   78.79%   -0.36%     
- Complexity      323      376      +53     
============================================
  Files            21       27       +6     
  Lines          2925     3414     +489     
  Branches        166      171       +5     
============================================
+ Hits           2315     2690     +375     
- Misses          541      651     +110     
- Partials         69       73       +4     

Impacted Files	Coverage Δ	Complexity Δ
...oogle/cloud/pubsub/v1/SubscriptionAdminClient.java	63.06% <ø> (ø)	52.00 <0.00> (ø)
...a/com/google/cloud/pubsub/v1/TopicAdminClient.java	58.95% <ø> (ø)	34.00 <0.00> (ø)
...google/cloud/pubsub/v1/stub/SchemaServiceStub.java	12.50% <12.50%> (ø)	1.00 <1.00> (?)
.../google/cloud/pubsub/v1/SchemaServiceSettings.java	17.94% <17.94%> (ø)	2.00 <2.00> (?)
...bsub/v1/stub/GrpcSchemaServiceCallableFactory.java	37.50% <37.50%> (ø)	3.00 <3.00> (?)
...om/google/cloud/pubsub/v1/SchemaServiceClient.java	60.91% <60.91%> (ø)	20.00 <20.00> (?)
...loud/pubsub/v1/stub/SchemaServiceStubSettings.java	86.69% <86.69%> (ø)	15.00 <15.00> (?)
...le/cloud/pubsub/v1/stub/GrpcSchemaServiceStub.java	93.43% <93.43%> (ø)	12.00 <12.00> (?)
...le/cloud/pubsub/v1/stub/PublisherStubSettings.java	83.95% <100.00%> (+0.39%)	21.00 <0.00> (ø)
...e/cloud/pubsub/v1/stub/SubscriberStubSettings.java	85.32% <100.00%> (ø)	28.00 <0.00> (ø)
... and 6 more

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[hongalex]

@hannahrogers-google @kamalaboulhosn This is this PR started to add schema support. It seems to be failing due to code coverage issues, could you take a look?