There's currently no support for creating application keys on Windows systems. This patch transitions the Windows key type to specifically refer to attestation keys, and reuses the existing wrapped key support for application keys. This allows the creation of keys in the platform store, while still allowing said keys to be manipulated with existing TPM functionality rather than duplicating it.

When generating a new key using a Windows TPM, a `wrappedKey20` was
returned, which couldn't be used for signing on Windows, as it's
backed by a `windowsTPM`. The `wrappedKey20` seems to be a type
specifically aimed at usage with a `wrappedTPM20`, which in turn
seems to be used on Linux and for testing, but not when instantiating
a TPM on Windows.

This commit adds the `newWindowsKey20` function, which returns
a key backed by a `windowsTPM`. The key is a `windowsAK20`,
now also conforming to the `key` interface, so that it can be used
for signing purposes.

My system is returning RCScheme if TPM_ALG_NULL is passed here. This should
be causing the key's default scheme to be used, but for some reason it
seems unhappy. Just explicitly set the scheme for now to avoid that.

This reverts commit 03018e6.