Fix signed URL and child emulator bugs

[cortadocodes]

Contents (#621)

Fixes

• Make Manifest.use_signed_urls_for_datasets idempotent
• Ensure directories exist before downloading datafile from URL
• Ensure child emulators can accept input manifests

[codecov-commenter]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (833cfe7) 94.38% compared to head (19cfd4c) 95.03%.
Report is 125 commits behind head on main.

Files	Patch %	Lines
octue/crash_diagnostics.py	93.58%	5 Missing ⚠️
octue/cloud/service_id.py	92.72%	4 Missing ⚠️
octue/cli.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #621      +/-   ##
==========================================
+ Coverage   94.38%   95.03%   +0.65%     
==========================================
  Files          78       75       -3     
  Lines        3436     3322     -114     
==========================================
- Hits         3243     3157      -86     
+ Misses        193      165      -28     

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