Expose Conditional Headers

Please consider exposing conditional headers `If-Modified-Since`, `If-Unmodified-Since`, `If-Match`,`If-None-Match`.

See https://docs.microsoft.com/en-us/rest/api/storageservices/specifying-conditional-headers-for-blob-service-operations for more info.

Start by locating how blobporter exposes or forwards request headers, then read the linked Azure documentation on conditional headers. Done means the four named headers—If-Modified-Since, If-Unmodified-Since, If-Match, and If-None-Match—are exposed with behavior matching the documented Azure Blob Storage operations.

Written by the indexing model from the issue text.