Support for custom headers in the download http requests and use ETags for upload

[aditya-07]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes #1989

Description

1. Added a way for users to specify http headers in DownloadWorkManager requests.
2. Update and Delete requests now use If-Match Http header with weak Etag.

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[jingtang10]

please add a network config to control if etag is used.

[jingtang10]

Thanks Aditya!