[FEATURE] Support implicit `@timestamp` field access in span

[yuancu]

Is your feature request related to a problem?

Currently, we have to specify a field to be used by the span function. E.g. source=my-index | stats count() by span(@timestamp, 1h)

What solution would you like?

For the ease of use, we'd like to make @timestamp a default field when users do not explicitly specify a field. I.e. we are going to support syntax like source=my-index | stats count() by span(1h), which will implicitly leverage @timestamp field.

What alternatives have you considered?

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Do you have any additional context?

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