Update firestore conformance tests

[BenWhitehead]

• Add dependency from com.google.cloud:google-cloud-firestore -> com.google.cloud:google-cloud-conformance-tests
• Remove proto files from com.google.cloud:google-cloud-firestore/tests in favor of those from com.google.cloud:google-cloud-conformance-tests
• Remove generated proto com.google.cloud.firestore.conformance.TestDefinition in favor of com.google.cloud.conformance.firestore.v1
• Remove com/google/cloud/firestore/conformance/test-suite.binproto
• Update ConformanceTest.java to use ConformanceTestLocator to load each
  of the 224 individual firestore test files.

Depends on #5541

[BenWhitehead]

@kolea2 Please Review

[codecov]

Codecov Report

Merging #5542 into master will increase coverage by 0.34%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #5542 +/- ##
============================================
+ Coverage 46.37% 46.71% +0.34% - Complexity 23246 24629 +1383 
============================================
Files 2352 2351 -1 Lines 251296 256127 +4831 Branches 29210 29325 +115 ============================================
+ Hits 116527 119648 +3121 - Misses 126746 127559 +813 - Partials 8023 8920 +897

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/cloud/dialogflow/v2/AgentsClient.java	47.72% <0%> (-9.27%)	19% <0%> (-2%)
.../google/cloud/dialogflow/v2beta1/AgentsClient.java	47.72% <0%> (-9.27%)	19% <0%> (-2%)
...va/com/google/cloud/compute/v1/InstanceClient.java	55.4% <0%> (-7.84%)	139% <0%> (+34%)
...ava/com/google/cloud/compute/v1/ProjectClient.java	57.07% <0%> (-6.99%)	55% <0%> (+13%)
...va/com/google/cloud/compute/v1/SnapshotClient.java	55.73% <0%> (-6.88%)	31% <0%> (+7%)
...oogle/cloud/kms/v1/KeyManagementServiceClient.java	62.88% <0%> (-6.84%)	103% <0%> (+25%)
...a/com/google/cloud/compute/v1/NodeGroupClient.java	60.36% <0%> (-6.82%)	51% <0%> (+12%)
.../com/google/cloud/compute/v1/SubnetworkClient.java	60.36% <0%> (-6.82%)	51% <0%> (+12%)
...d/compute/v1/RegionInstanceGroupManagerClient.java	54.5% <0%> (-6.79%)	51% <0%> (+12%)
.../com/google/cloud/compute/v1/TargetPoolClient.java	57.07% <0%> (-6.73%)	47% <0%> (+11%)
... and 616 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update da86687...c745bd3. Read the comment docs.

[BenWhitehead]

The builds are currently failing because I need to rebase them. Since this PR is dependent on another PR I'll wait to rebase until that PR is merged. The changes in this PR can still be reviewed before the rebase happens.