Stub "Transcode Complete" callback

[thomshutt]

This is so we can do any final iterations on how the output will look and slightly unblock Studio

[codecov]

Codecov Report

Merging #35 (8821562) into main (1753b3b) will decrease coverage by 6.35473%.
The diff coverage is 0.00000%.

@@                 Coverage Diff                 @@
##                main         #35         +/-   ##
===================================================
- Coverage   51.07914%   44.72441%   -6.35474%     
===================================================
  Files              8           8                 
  Lines            556         635         +79     
===================================================
  Hits             284         284                 
- Misses           235         314         +79     
  Partials          37          37                 

Impacted Files	Coverage Δ
clients/callback_client.go	63.06306% <0.00000%> (-14.71473%)	⬇️
handlers/handlers.go	29.50000% <0.00000%> (-12.04931%)	⬇️

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Impacted Files	Coverage Δ
clients/callback_client.go	63.06306% <0.00000%> (-14.71473%)	⬇️
handlers/handlers.go	29.50000% <0.00000%> (-12.04931%)	⬇️

[thomshutt]

@victorges I'm going to merge this now all the comments are addressed, just to avoid conflicts with other in-flight PRs but can change anything else you want in a followup PR