Email/set create bodyStructure support

https://jmap.io/spec-mail.html#emailset

In addition to the helpers for composing classic multipart emails (alternative text/plain + text/html and attachments), RFC-8621 allows also to create emails by specifying their bodyStructure:

If a bodyStructure property is given, there MUST NOT be textBody, htmlBody, or attachments properties.

If given, the bodyStructure EmailBodyPart MUST NOT contain a property representing a header field that is already defined on the top-level Email object.

Specifying bodyStructure within Email/set create entries is so far not supported by James.

How

• EmailCreationRequest needs to have a bodyStructure field
• EmailCreationRequest::toMime4JMessage needs to handle the cases when bodyStructure is specified.

Definition of done:

Integration tests for Email/set create bodystructure, referencing both blobs (via blobId) and bodyValues.