[Testing] Populate AppHost environment from launch profile and support launch profile override

[ReubenBond]

Description

Fixes #5093

This PR simulates launch profile loading for the testing host. It does not mutate the process' environment since that could cause issues with concurrently running tests. Instead, it propagates the environment variables specified in the AppHost's launch profile to the AppHost's configuration.

This PR sets the DOTNET_LAUNCH_PROFILE environment variable to the name of the first launch profile if not overridden. It does not consider the DOTNET_LAUNCH_PROFILE from the test runner's environment.

This re-applies #7363, which was reverted in #7371

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • No
• Does the change make any security assumptions or guarantees?
  • No
• Does the change require an update in our Aspire docs?
  • No