Add Functions to Read and Write `dcz` Headers

[felixhandte]

This PR adds support to Zstd for handling the header used by the dcz HTTP Content-Encoding, described in draft-ietf-httpbis-compression-dictionary.

To-Do:

• [x] SHA-256 Impl.
• [x] SHA-256 Tests.
• [x] User API.
• [x] User API Impl.
• [x] User API Tests.
• [ ] CLI Support.
• [ ] CLI Tests.