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.

…tches

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.

On Windows, when the key is managed by the OS, keys are stored on
the filesystem. When trying to create a key with the same name, this will
fail with the following error:

`NCryptCreatePersistedKey returned 8009000F: The operation completed successfully.`

This commit adds support for deleting these keys, so that a new
key can be created with the same name.

Have only tested this on Windows so far. My assumption is that for
keys created with `go-attestation` on Linux, the keys aren't persisted
"inside the TPM", so there's nothing to do when deleting them, except
for any keys managed externally.