Keep policy information encrypted too

[v0lkan]

SPIKE's current trust model treats the backing store as "untrusted".

Which means anything in the backing store that is sensitive or can be used as an attack vector shall be encrypted.

We already encrypt secrets in the backing store;
however policies are stored in plain text.

The spiffe_id_pattern and path_pattern of a SPIKE policy (although much less sensitive than a secret) can be used by and adversary as a data point to devise attacks against SPIKE Nexus. Thus they should be encrypted too.