Disable cert revocation check if cert validation is disabled

[ZekeSnider]

This addresses an issue described in #664. It makes it so that certificate revocation is not checked if certificate validation is disabled on http_client_config. Previously there was no way to disable this functionality.

[kobykahane]

@ZekeSnider In my opinion, a complete solution to issue #664 would also allow disabling certificate revocation checking without completely disabling certificate validation as well.

[msftclas]

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