Sync FindCrypto. Add option to force crypto linkage

[sbiscigl]

Description of changes:

Syncs FindCrypto from S2N, and adds option to force usage of static or dynamic crypto.

The reason these options are being introduced is to force the SDK to use a static or dynamic libcrypto independent of BUILD_SHARED_LIBARIES. Currently if BUILD_SHARED_LIBARIES is set to OFF there is no way to force the SDK to use a dynamic libcrypto.

running with something like

cmake -DBUILD_ONLY="s3" \
  -DBUILD_SHARED_LIBS=OFF

will result in static crypto being chosen over dynamic if dynamic is present

cmake -DBUILD_ONLY="s3" \
  -DBUILD_SHARED_LIBS=OFF \
  -DAWS_USE_CRYPTO_SHARED_LIBS=ON \
  -DS2N_USE_CRYPTO_SHARED_LIBS=ON

will result on dynamic crypto being used even though we are building statically

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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