Call find_dependency(CURL) if the Curl client is enabled.

[teo-tsirpanis]

Description of changes:

This PR updates the exported config file to call find_dependency(CURL) if the Curl client had been enabled when building the SDK.

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[jmklix]

Why exactly do you need this added to the cofig file? This looks like an ok change, but we want to make sure we understand what your use case is

[teo-tsirpanis]

aws-cpp-sdk-core sometimes depends on the CURL::libcurl target but until now, that target is not guaranteed to exist. By finding Curl when we find the AWS SDK, we guarantee that the Curl targets exist if needed.