Add Creation of SSLPrivateKey from UnexportableSigningKey on Windows

Made a small refactoring in crypto/unexportable_key_win to allow
reusing the same provider+key initialization logic from a wrapped
TPM-backed NCrypt key.

Added a new function in ssl_platform_key_win which return an
SSLPrivateKey when given a UnexportableSigningKey instance. Since both
of these instances need their own NCRYPT_KEY_HANDLE, this function
effectively uses loads the key again using the wrapped value.

Bug: b:326086170
Change-Id: I5f7c4d295da4f30d0e445a94268ecb738fc6c165
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5344872


Commit-Queue: Sébastien Lalancette <seblalancette@chromium.org>
Reviewed-by: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281141}