Recalculate signing time for every retry attempt (#2643)

## Motivation and Context
When SigV4 signing was ported over to the orchestrator, the request time
got calculated once before the retry loop, which was incorrect. This PR
moves that request time calculation into the request signer so that it
happens on every attempt (unless there is a configured request time
override).

This PR also refactors auth to use the `ConfigBag` instead of a separate
signing properties `PropertyBag`.

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