timeseriesinsights - --timestamp-property-name should be made optional

[riserrad]

• If the issue is to do with Azure CLI 2.0 in-particular, create an issue here at Azure/azure-cli

Extension name (the extension in question)

timeseriesinsights

Description of issue (in as much detail as possible)

When creating an Event Source, the --timestamp-property-name should be an optional parameter.

Please let us know if there's something on the Service's swagger side that needs to be reviewed to make this possible.

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[yonzhan]

add to S170