Allow signing with custom timestamp

[ThrawnCA]

Is your feature request related to a problem? Please describe.

Signed URLs to static resources like S3 objects are hard to cache, because they change every time. There is a suggested approach at https://advancedweb.hu/how-to-use-s3-signed-urls-with-cloudfront/ but it relies on monkey-patching the system time function(s), which can lead to race conditions when running multiple threads.

Describe the solution you'd like

We would like to be able to pass a custom timestamp to the functions in botocore/signers.py. This would allow us to eg round the current time down to the previous half-hour, generate an S3 pre-signed URL from that with 1-hour expiry, send it to the client, and cache the result in CloudFront. That way, for a half-hour at a time, the same pre-signed URL will be generated each time, and CloudFront can serve the object from cache.

[ThrawnCA]

We've found an alternative solution to this, by generating a signature with the current time but then caching it for the desired length. On the other hand, that requires an external cache (we're using Redis), so there is still a use case for this feature.

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[ThrawnCA]

We've found an alternative solution for the majority of this, but it may still have a use.

[robbat2]

This should be great for another use case: easier mocking time for the testing Boto behavior. Solutions like freezegun exist, but they aren't always compatible with other python modules. Being able to inject a specific time for Boto signing would greatly improve ease of use.