Add timesource abstration to aws-smithy-async (#2721)

## Motivation and Context
- Controlling time is required for several testing use cases
- #2087 
- #2262 

## Description
Introduce `TimeSource` trait, a real implementation, and a test
implementation.

## Testing
These changes are used in the timestream PR

## Checklist
No changelog, these changes have no impact since the code is not yet
utilized

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