Persist tokens and other outputs securely for test execution

[heaths]

Output tokens from test-sources.json need to be stored securely so we can remember them across sessions. We're opting to use this instead of environment variables, since changing environment variables requires restarting IDEs like VS (which spawns many child processes, most of which don't respond to broadcast messages asking them to update their environment variables).

• On Windows, we'll use DPAPI to encrypt files under %AppData%\azuresdk.
• On linux and MacOS, we'll chmod 700 ~/.local/azuresdk.

On both systems, the subdirectory will be a hash of the repo root (case-sensitive on linux/MacOS; tolower'd in Windows).

[heaths]

@pakrym did this for Windows, but we don't yet do this for linux and should consider it.