[Synthetics] Add synthetics ci endpoint

[api-clients-generation-pipeline]

See DataDog/datadog-api-spec#510

[therve]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #429 into master will decrease coverage by 0.23%.
The diff coverage is 8.17%.

Impacted Files	Coverage Δ
api/v1/datadog/model_synthetics_basic_auth.go	0.00% <0.00%> (ø)
...pi/v1/datadog/model_synthetics_ci_test_metadata.go	0.00% <0.00%> (ø)
...v1/datadog/model_synthetics_ci_test_metadata_ci.go	0.00% <0.00%> (ø)
...1/datadog/model_synthetics_ci_test_metadata_git.go	0.00% <0.00%> (ø)
api/v1/datadog/model_synthetics_test_request.go	8.37% <0.00%> (ø)
..._synthetics_trigger_ci_tests_response_locations.go	0.00% <0.00%> (ø)
...el_synthetics_trigger_ci_tests_response_results.go	2.12% <2.12%> (ø)
api/v1/datadog/model_synthetics_ci_test_.go	2.68% <2.68%> (ø)
...adog/model_synthetics_trigger_ci_tests_response.go	5.19% <5.19%> (ø)
api/v1/datadog/model_synthetics_ci_test_body.go	11.62% <11.62%> (ø)
... and 9 more