Drop `digestMultibase` in favour of simply `digest`

[filip26]

Description

Building on the proposal to sign requests transparently without decoding the multibase value (as outlined in the related issue regarding digestMultibase expectations digitalbazaar/hmbd#5), we should consider dropping the digestMultibase field entirely in favour of a digest field.

Rationale

• Redundancy: If the witness service signs the payload transparently without decoding or interpreting the multibase encoding, the multibase wrapper provides no functional benefit at this layer.
• Simplicity: A standard digest field aligns directly with standard cryptographic workflows and simplifies the request schema.
• Interoperability: Removing multibase-specific requirements reduces the dependency footprint for clients and systems interacting with the service.

Proposed Changes

• Replace the digestMultibase property with digest in the specification and schema.
• Update signing and verification logic to process the digest value directly.