Place generated files in CMAKE_BINARY_DIR.

[teo-tsirpanis]

Issue #, if available:

Description of changes:

Note

This is a reopening of #3459, which was closed for reasons unknown to me.

This PR updates the CMake project to place generated files in the binary directory, instead of the source directory. This follows best practices, and unblocks the vcpkg port to support configuring aws-sdk-cpp in parallel.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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