Fix build without default-features and configure digest hash functions

[DXist]

Three contributions:

• configure the established two-space indentation configuration for rustfmt and editor. In my personal opinion, it's better to use default 4 spaces for an opensource project.
• build without default features compiles (currently, it always imports -Sync items)
• It's possible to configure supported SHA digests via digest-sha256 and digest-sha512. The primary benefit of canonicalization of a single allowed digest function is to support persisting computed digests into the idempotency keys table to validate the persisted digest on request retries.