Add support for custom PUT HTTP headers in cache archiver

This commit adds a new method in the cache adapter to allow for custom, unsigned HTTP headers. These headers are passed via the --header parameter to the cache archiver process. The default behavior of adding Content-Type and Last-Modified headers is retained if the --header parameters is omitted.

This is in preparation for supporting the Azure PUT Blob API (https://docs.microsoft.com/en-us/rest/api/storageservices/put-blob), which needs the x-ms-blob-type HTTP header, which cannot be sent via a query string.