refactor: add separate policy folder

[lrstewart]

Release Summary:

Description of changes:

Proposal: start moving the logic for security policies out of "tls". "security policy" is an s2n-tls concept created to make configuration easier, so I'd argue it doesn't belong in "tls". Granted, other things also probably don't belong in "tls", but creating new source folders is hard, as evidenced by this diff.

I'd like to move the definitions for the components of the security policy (like cipher_preferences, signature_preferences, etc.) into the same folder. Also the actual list of security policies / component definitions (but that might break some internal processes...).

Callouts:

Some of the build/test logic I added "policy" to are probably deprecated and unnecessary. However, I argue that determination is out of scope, and "policy" should just appear everywhere our current folders do for simplicity.

Testing:

All builds should continue to work in CI.

I made this change manually, then also asked Amazon Q to exhaustively search all files and add the "policy" folder anywhere that already used the "error" and "stuffer" folders. Its results agreed with mine.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.