[Waiter] Data Upload Checksum

[otaviof]

Using the BuildSource type LocalCopy injects a waiter container in order to stream local data to the build pod, where currently the user can inform a timeout (via .spec.sources[].timeout).

We should improve this process and verify the checksum of the payload streamed, thus we can rule out any inconsistencies.

Some initial open questions I would like to share:

• How the CLI can (reliably) calculate the payload digest?
• How the CLI an inform the digest to the waiter?
• How the waiter will verify the checksum?

[qu1queee]

From grooming, the problem to tackle is to be able to understand when the data was done with the streaming, without relying on the waiter with timeout. Keeping this one for triage.