Directory+File: add withTimestamps for normalizing file/dir timestamps

[vito]

fixes #4104

Adds the following API:

extend type Directory {
  "This directory with all file/dir timestamps set to the given time, in seconds from the Unix epoch"
  withTimestamps(timestamp: Int!): Directory!
}

extend type File {
  "This file with its created/modified timestamps set to the given time, in seconds from the Unix epoch"
  withTimestamps(timestamp: Int!): File!
}

Bikeshedding opportunities:

• withTimestamps name
• timestamp param name
• timestamp param type (should we have a UnixSeconds scalar? or just rename the param to unixSeconds?)

In hindsight there might be a less confusing name - we've got a plural withTimestamps yet a singular timestamp param. The name is plural because it modifies the contents recursively to a single timestamp.

One caveat: when mounting the Directory into a container the mountpoint itself won't have the desired timestamp. So if you do tar -cf - -C /dir . you'll still end up with an unstable tar archive because of the timestamp on /dir. I tried a few tricks with LLB but none of them worked. The workaround in this case is to just target the contents instead (* instead of .).

[vito]

I sunk some time into turning timestamp: Int! into a proper date scalar represented by time.Time in Go, Date in JS, and datetime in Python, but I ended up backing out for now as the complexity is pretty high. I can dive back into it though if we're not happy with Int.

[sipsma]

LGTM