Declarative description of one provider's HMAC signature scheme.
The one field with no sensible default is signature_header: which header
carries the signature(s). Everything else defaults to the most common shape
(raw SHA-256 hex over the body).
Summary
Types
@type encoding() :: :hex | :base64
@type hash() :: :sha256 | :sha512 | :sha1
@type parse() :: :whole | :csv_pairs | :space_versions
@type secret_decode() :: :raw | :whsec_base64