fix: lazily resolve proxy environment variables

[lauzadis]

This PR updates the EnvironmentProxySelector to lazily resolve environment variables rather than immediately parsing them during initialization

Issue #

Resolves awslabs/aws-sdk-kotlin#1281

Description of changes

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ianbotsf]

Nit: Can we add a new test verifying that junk values in the vars/props are ignored when overriding the proxy selector in HTTP client config?

[lauzadis]

@ianbotsf I added a test but it required making EnvironmentProxySelector public with an @InternalApi annotation because the tests are in a separate module, do you think it's worth keeping this way?