Require `content-digest` header in requests with body

[wilsonianb]

https://datatracker.ietf.org/doc/html/draft-ietf-gnap-core-protocol#section-7.1.1

When the message contains a request body, the covered components MUST
also include the following:

"content-digest": The Content-Digest header as defined in
[I-D.ietf-httpbis-digest-headers]. When the request message has a
body, the signer MUST calculate this field value and include the
field in the request. The verifier MUST validate this field
value. REQUIRED when the message request contains a message body.