Add on-call route for creating an email for a specific user

[api-clients-generation-pipeline]

See DataDog/datadog-api-spec#4547 Test branch datadog-api-spec/test/dd/add-email-component-on-call-spec

[datadog-datadog-prod-us1]

⚠️ Tests

⚠️ Warnings

🧪 2 Tests failed

    Create an On-Call email for a user returns "Bad Request" response from On-Call at features/v2/on-call.feature

Create an On-Call email for a user returns "Bad Request" response from On-Call at features/v2/on-call.feature (Datadog)

Cassette '/home/runner/work/datadog-api-client-ruby/datadog-api-client-ruby/cassettes/features/v2/on-call/Create-an-On-Call-email-for-a-user-returns-Bad-Request-response.yml' not found: create one setting \`RECORD=true\` or ignore it using \`RECORD=none\`

ℹ️ Info

❄️ No new flaky tests detected

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