Add C2PA signing

[leszko]

Add C2PA signing to the FFMPEG pipeline.

How does sample output videos look like?

• Output video when the source video is C2PA signed
• Output video when the source video is not C2PA signed

You can download these videos and verify in the C2PA online verify tool: https://contentcredentials.org/verify

Note that this PR

• Adds signing only for the FFMPEG pipeline, not yet for Mediaconvert pipeline
• Adds c2pa.published assertion done by Livepeer Studio, not yet c2pa.transcoded
• Adds C2PA signature only to the output mp4 files, not yet to HLS

Detailed changes

• Add c2pa package which uses c2patool tool to sign videos
• Add two flags c2pa-private-key and c2pa-certs to define paths to PK and Certs used for C2PA signature
• Add c2pa bool param to the VOD request to decide if the output mp4 videos should be signed
• Change the logic of copyFileToLocalTmpAndSegment() not to delete the local copy of the source file, because it's needed to preserve the source signature

[codecov]

Codecov Report

Merging #931 (c28ebb3) into main (181d693) will decrease coverage by 0.37529%.
The diff coverage is 8.06452%.

@@                 Coverage Diff                 @@
##                main        #931         +/-   ##
===================================================
- Coverage   51.24981%   50.87452%   -0.37529%     
===================================================
  Files             63          64          +1     
  Lines           6521        6575         +54     
===================================================
+ Hits            3342        3345          +3     
- Misses          2907        2957         +50     
- Partials         272         273          +1     

Files	Coverage Δ
config/cli.go	53.95683% <ø> (ø)
handlers/upload.go	69.26829% <100.00000%> (+0.15064%)	⬆️
pipeline/coordinator.go	77.24289% <0.00000%> (-0.68205%)	⬇️
transcode/transcode.go	36.84211% <0.00000%> (-0.46753%)	⬇️
pipeline/ffmpeg.go	35.23132% <22.22222%> (-0.69461%)	⬇️
c2pa/c2pa.go	0.00000% <0.00000%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 181d693...c28ebb3. Read the comment docs.

Files	Coverage Δ
config/cli.go	53.95683% <ø> (ø)
handlers/upload.go	69.26829% <100.00000%> (+0.15064%)	⬆️
pipeline/coordinator.go	77.24289% <0.00000%> (-0.68205%)	⬇️
transcode/transcode.go	36.84211% <0.00000%> (-0.46753%)	⬇️
pipeline/ffmpeg.go	35.23132% <22.22222%> (-0.69461%)	⬇️
c2pa/c2pa.go	0.00000% <0.00000%> (ø)