Sparse mode: handle documents without their actual content

[cinnamon-bun]

Document signatures should depend on the hash of the content, not the content itself.

This would let you:

• Download documents without their content, just metadata, and still verify signatures
• Locally delete content but keep metadata (e.g. when removing blocked content).

Add contentHash field

• Add required contentHash field to documents
• Compute the hash when writing a document: sha256(content)
• Use contentHash and not content when hashing a document
• When verifying document, check the hash matches the content

Allow content to be null

• Allow content to be null, meaning it's missing (note, empty documents are still content: "")
• When verifying document, check the hash matches the content OR the content is null
• Add query option includeContent: boolean (include or omit the content itself in the returned docs)
• Add query option hasContent: boolean (find docs with missing content, or existing content)

[cinnamon-bun]

Content can't be null yet, otherwise this is done in e41f5c7

Allowing content to be null might change the API a bit -- I want to think about that