Add HCL function to return Unix timestamp

[benjamb]

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Description

With the old json engine, I could use {{ timestamp }} to get a Unix timestamp, the new timestamp() HCL function does not behave the same way, neither is there any way to translate that date time into a Unix timestamp (e.g. using `formatdate()).

Presently I have to pass this in as a variable, which quickly becomes tedious.

Potential References

zclconf/go-cty#130