function/parse_duration: Add duration parsing function

[ricardbejarano]

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[ricardbejarano]

First contribution to this provider, so apologies if there are any errors of form.

This brings Go's time.ParseDuration function to Terraform, allowing users to define durations as "1h30m" instead of 60*90, which is much more readable.

Most of the code has been adapted from the rfc3339_parse function.

If it only goes up to hours in resolution is because that's what Rob Pike said makes sense.

[ricardbejarano]

@austinvalle ping!

[ricardbejarano]

Any updates? Can we get this merged?