Ability to provide headers in the DownloadWorkManager

[anchita-g]

Is your feature request related to a problem? Please describe.
Application should be able to use e-tags based headers like if-modified-since, if-none-match for optimised download/upload of resources.

Describe the solution you'd like
Along with the URL, it will be useful to provide the ability to mention the additional headers that should be sent in the request to the server.

Additional context
FHIR specification

cc: @jingtang10

[santosh-pingle]

@anchita-g
Are you going to work on this issue?
(If yes, please add priority and effort to fix this issue.)