Fix: Propagate context with DownloadContents

[HakShak]

Main motivation is tracing as this can be the most time consuming operation.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 92.90%. Comparing base (2b8c7fa) to head (c29514f).
Report is 67 commits behind head on master.

Files	Patch %	Lines
github/repos_contents.go	50.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@ Coverage Diff @@## master #3181 +/- ##
==========================================
- Coverage 97.72% 92.90% -4.82% 
==========================================
Files 153 171 +18 Lines 13390 11548 -1842 ==========================================
- Hits 13085 10729 -2356 - Misses 215 726 +511 - Partials 90 93 +3 

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[gmlewis]

Thank you, @HakShak !
LGTM.

Awaiting second LGTM+Approval from any other contributor to this repo before merging.

[tomfeigin]

LGTM

[gmlewis]

Thank you, @tomfeigin !
Merging.