Allow integration tests to run in both runtime modes (#2709)

## Description
This will allow integration tests under `aws/sdk/integration-tests` to
run in both smithy runtime modes. Prior to the PR, the integration tests
use `map_operation` to customize operations, which is not supported in
the orchestrator, preventing us from running them in the orchestrator
mode. Fortunately, all the usages of `map_operation` in the integration
tests involve setting a test request time and a test user agent in a
property bag.

This PR stops using `map_operation` in those tests and instead
introduces separate test helper methods in both runtime modes: one for
setting a test request and the other for setting a test user agent. They
allow the integration tests to compile and run in both modes.

Note that the integration tests in the orchestrator do not necessarily
pass at this time. We'll address the failures subsequently.

## Testing
Confirmed integration tests continue passing with the changes when run
in the middleware . Confirmed those complied (but not necessarily
passed) in the orchestrator.

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Co-authored-by: Yuki Saito <awsaito@amazon.com>