Windows: Workaround for storing >= 4k (4096 bit) client-cert SSL keys

Windows: Workaround for storing >= 4k (4096 bit) client-cert SSL keys

With QtKeychain on Windows, storing larger keys in one keychain entry causes the
following error due to limits in the Windows APIs:
  Error: "Credential size exceeds maximum size of 2560"

To avoid overhead on the other platforms and balance code duplication, this
approach puts some read- and write-parts into Windows-only defines.

For reference also see previous fixes:
- https://github.com/nextcloud/desktop/pull/1389
- https://github.com/nextcloud/desktop/pull/1394

This (again) fixes the re-opened issue:
- https://github.com/nextcloud/desktop/issues/863

Signed-off-by: Michael Schuster <michael@schuster.ms>