Add features to choose crypto backend

[nilskch]

Before this PR

Currently the default crypto backend is aws-lc-rs. This backend is not cross compilable from linux -> mac (issue) and the maintainers are not planning to implement support for this. We need conjure-runtime to be cross compilable because we are building a CLI app that runs on a variety of different platforms.

After this PR

This PR adds the features ring and aws-lc-rs (default) which let you choose the CryptoProvider. ring is cross compilable.

==COMMIT_MSG==
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Possible downsides?

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• improvement: An incremental improvement in the functionality or operation of the service.
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Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Add features to choose crypto backend #226

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