default HTTPS stack based on platform support

[aajtodd]

Motivation and Context

• Hyper Upgrade #1925
• [tracking] support hyper/http/etc. 1.0 awslabs/aws-sdk-rust#977

Description

Modifies the default TLS provider based on platform support and changes the default for unix to s2n-tls.

I had to introduce a new crate aws-smithy-default-tls to make this work. This is because we can't modify dependencies or add new features based on a feature. Doing it without a new crate seems only possible with rust-lang/cargo#1197.

The way around this is to consider the crate itself a "feature" and then we can make use of rust-lang/cargo#7914 which allow specifying features on a dependency predicated on target.

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• AWS SDK (ignoring whitespace)
• Client Test (ignoring whitespace)
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

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