Test: replace Portable.BouncyCastle with System.Formats.Asn1

[dtivel]

Bug

Fixes: https://github.com/NuGet/Client.Engineering/issues/969

Description

This change replaces our Portable.BouncyCastle test dependency with types that leverage System.Formats.Asn1 functionality.

BouncyCastle's alternate implementation of many cryptographic types provided a good baseline for our package signing tests. To remove BouncyCastle, I

• mostly recreated similar functionality using System.Formats.Asn1
• copied a few implementations from dotnet/runtime
• removed test coverage in a few situations

With the exception of Directory.Packages.props, this PR only modifies files under /test.

Occasionally, I opportunistically also fixed IDE warnings/errors for unused code or format rule violations, such as constants with a leading _ or unused using statements.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests N/A
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc. N/A

CC @aortiz-msft, @kartheekp-ms