Add `head` property to `FileDownloadDelegate`'s `Progress`/`Response` struct #80

Job	Run time
Cxx interop / Construct Cxx interop matrix	4s
Unit tests / Construct unit test matrix	4s
Soundness / API breakage check	4m 12s
Soundness / Broken symlinks check	4s
Soundness / Documentation check	2m 50s
Soundness / Format check	34s
Soundness / License headers check	5s
Soundness / Unacceptable language check	11s
Soundness / Python lint check	4s
Soundness / Shell check	47s
Soundness / YAML lint check	4s
Cxx interop / Cxx interop / Linux (5.9)	2m 23s
Cxx interop / Cxx interop / Linux (5.10)	2m 24s
Cxx interop / Cxx interop / Linux (6.0)	2m 37s
Cxx interop / Cxx interop / Linux (nightly-6.1)	3m 11s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 15s
Unit tests / Unit tests / Linux (5.9)	4m 40s
Unit tests / Unit tests / Linux (5.10)	4m 42s
Unit tests / Unit tests / Linux (6.0)	5m 2s
Unit tests / Unit tests / Linux (nightly-6.1)	5m 48s
Unit tests / Unit tests / Linux (nightly-main)	5m 30s
	48m 31s

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