Decide what headers to configure by default when using FS-like API

[cdata]

After #37 is resolved, we will have a relatively high-level FS-like API for writing files to - and reading files from - a Sphere. The write side of this API will allow the user to configure auxiliary headers to be included in the wrapping memo. But, for a high-level API like this we will probably also want to pick some implicit default headers. Examples of this may include:

• DID of the author
• DID of the sphere
• Author's cryptographic signature of body CID (noting that we already sign the sphere on each revision)
• Write time
• Others?