update attestation schema

[Arsh-Sandhu]

• remove digestAlgo attribute from attestation schema
• add edge section in attestation schema

[nkongsuwan]

@Arsh-Sandhu

• According to my discussion with Daniel, the digest field’s name should be kept as “digest” so that the credentials are more flexible, not limited to just xsaids.
• The edge section should be copied from https://github.com/provenant-dev/public-schema/blob/main/award/award.schema.json#L135. This makes the schema flexible such that the credentials can be chained to non-vLEI credentials.

[Arsh-Sandhu]

@nkongsuwan updated schema to use digest attribute instead of xsaid and changed edge section