Remove Bouncy Castle dependency when targeting .NET 8 et al.

[teo-tsirpanis]

Description

This PR updates the CloudFront and EC2 extension libraries to import private keys from PEM with the relatively new ImportFromPem method when targeting .NET 8. This lets us remove the dependency to Bouncy Castle in this case.

Motivation and Context

Removing third-party dependencies is always a good thing to do, especially when the dependency is a cryptographic library, and the replacement is trivial.

Testing

Unit tests succeed locally.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license

[peterrsongg]

Thanks for the PR and for cleaning up our extension packages! Ran the tests myself, and it looks good. Do you mind doing a version bump to 4.0.0.1 for this?

You would have to update this file for EC2 and this file for cloudfront.