[sbiscigl]

Description of changes:

This is the first set of changes for 1.12, the next major version of the AWS SDK for C++. Every 1.12 change is staged on the release/1.12 branch, and merging that branch with main is what releases 1.12. This PR changes the SDK's defaults.

• C++17 is the default and minimum standard, set through CMake's CMAKE_CXX_STANDARD. Setting CPP_STANDARD now fails the build.
• The minimum CMake version goes from 3.13 to 3.17.
• The CRT HTTP client is the default transport, with non-blocking async I/O on. libcurl and WinHTTP still build with -DUSE_CRT_HTTP_CLIENT=OFF.
• The CRT uses its own crypto by default instead of the system's OpenSSL.
• Each client's default executor is a PooledThreadExecutor sized to the machine's hardware threads.
• DefaultExecutor is renamed UnboundedExecutor, and the old name still compiles as a deprecated alias.
• Each request's user agent names its client's executor as md/executor#.
• Generated clients call the executor factory once during init instead of twice.
• A new toolchain policy lists the minimum GCC, Clang, Apple Clang, MSVC and CMake versions

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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