Add assertion types for DNS Synthetics tests

[api-clients-generation-pipeline]

See DataDog/datadog-api-spec#580

[jirikuncar]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #486 into master will decrease coverage by 0.25%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
api/v1/datadog/model_synthetics_assertion_type.go	0.00% <ø> (ø)
api/v1/datadog/api_users.go	9.49% <0.00%> (-23.62%)	⬇️
api/v1/datadog/model_user.go	0.90% <0.00%> (-13.52%)	⬇️
api/v1/datadog/client.go	21.91% <0.00%> (-5.98%)	⬇️
api/v1/datadog/model_access_role.go	30.00% <0.00%> (-5.00%)	⬇️
api/v1/datadog/model_user_response.go	0.00% <0.00%> (-3.45%)	⬇️
api/v1/datadog/utils.go	4.54% <0.00%> (-1.14%)	⬇️
api/v1/datadog/model_user_list_response.go	3.44% <0.00%> (+3.44%)	⬆️