Extend Zarr entry upload procedure to support client-assigned Content-Types

[jwodder]

(See dandi/dandidav#90 for motivation)

Feature request: Support an optional content_type: str | null field in the objects in POST /zarr/{zarr_id}/files/ requests. When this field is present and non-null, the signed upload URL that the Archive produces for the entry should set the Content-Type field to the given value so that GET requests for the resulting S3 object will be replied to with responses that have that Content-Type.

Question: When the client performs the actual upload to the signed URL, must it specify the same Content-Type header again?