Add `head` property to `FileDownloadDelegate`'s `Progress`/`Response` struct #82

Job	Run time
Cxx interop / Construct Cxx interop matrix	3s
Unit tests / Construct unit test matrix	3s
Soundness / API breakage check	4m 18s
Soundness / Documentation check	2m 53s
Soundness / Broken symlinks check	3s
Soundness / License headers check	4s
Soundness / Format check	39s
Soundness / Unacceptable language check	2s
Soundness / Python lint check	11s
Soundness / Shell check	31s
Soundness / YAML lint check	7s
Cxx interop / Cxx interop / Linux (5.9)	2m 22s
Cxx interop / Cxx interop / Linux (5.10)	2m 15s
Unit tests / Unit tests / Linux (5.9)	4m 42s
Unit tests / Unit tests / Linux (5.10)	4m 41s
Cxx interop / Cxx interop / Linux (6.0)	2m 42s
Cxx interop / Cxx interop / Linux (nightly-6.1)	3m 17s
Unit tests / Unit tests / Linux (6.0)	4m 56s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 16s
Unit tests / Unit tests / Linux (nightly-6.1)	5m 37s
Unit tests / Unit tests / Linux (nightly-main)	5m 33s
	48m 15s

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