[WIP] Sealed state, or encrypted state at rest

[hargettp]

Inspired by Ansible's vault files, this feature enables encrypted state for the local backend. The intent is to provide a roughly analogous feature. The feature is intended to enable reasonably secure sharing of state through Git repositories within small teams. Keys may be shared externally but generally stored in consistent locations on team members machines outside of the Git repository containing sealed state.

The changes include:

• New configuration options for the local backend
• Additional ReadSealedState and WriteSealedState functions for reading and writing encrypted state; while not implemented here, these same functions may be appropriate for implementing sealed state in other backends (e.g., S3)
• Updates to documentation for the website
• A unit test to ensure basic roundtrip encryption / decryption works

NOTE: I am not a cryptography expert. All code written here is an attempt to emulate good patterns that already exist. The implementation depends on keys generated with a password-based key derivation function and AES-256.

While not as complex as the features requested hashicorp#16066 or hashicorp#9556, it is intended to enable the basic scenario for sharing encrypted state.