Add CMakeLists.txt for _CryptoExtras (#420)

[Akumar1313]

[One line description of your change]

Checklist

• I've run tests to see all new and existing tests pass
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

If you've made changes to gyb files

• I've run ./scripts/generate_boilerplate_files_with_gyb.sh and included updated generated files in a commit of this pull request

Motivation:

The Swift toolchain bootstrap builds SwiftPM using CMake, and SwiftPM depends on swift-certificates, which in turn depends on _CryptoExtras. Every module in swift-crypto has a CMakeLists.txt except _CryptoExtras, causing the CMake bootstrap to fail with a missing module error.

Fixes #420.

Modifications:

Added Sources/_CryptoExtras/CMakeLists.txt that declares the _CryptoExtras library target with its single source file (Exports.swift), links it against CryptoExtras, and registers it in the SWIFT_CRYPTO_EXPORTS property.
Added add_subdirectory(_CryptoExtras) to Sources/CMakeLists.txt after CryptoExtras to respect the dependency ordering.

Result:

The _CryptoExtras module is now discoverable by CMake, allowing swift-certificates (and any other downstream CMake consumer) to resolve its dependency on _CryptoExtras during the toolchain bootstrap build.

[Akumar1313]

@Lukasa can you review this pr and lmk if this change help with CMake for _CryptoExtras.

[Lukasa]

Nice, thanks!