Make our load-test client speak 🗣️ 🪄

[streamer45]

Summary

PR adds a couple of functionalities to our load-test client:

• A new -speech-file flag to pass any valid audio Opus encoded file. I am including two generated speech samples with their text source.
  • This could be useful to measure transcription quality and performance of well known samples.
• A way to generate speech from text on the fly and essentially script conversations. See lt/cmd/speech/main.go.
  • This allows for a more flexible way to test realistic conversations between multiple users.

Had to restructure things a bit to make this work so updating the docs as well.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9477c5a) 9.36% compared to head (1e7d3bf) 9.36%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #614   +/-   ##
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  Coverage   9.36%   9.36%           
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  Files         26      26           
  Lines       5308    5308           
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  Hits         497     497           
  Misses      4759    4759           
  Partials      52      52           

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[streamer45]

Made ./lt a separate Golang module because it actually didn't make sense to share dependencies with the plugin, even more so now that we have some CGO stuff in there (opus).

[cpoile]

Approving because it works and I've made enough changes/additions on my side that to make any here would cause some bad conflicts.
Thank you again for this, truly helpful for the live captions testing.

[streamer45]

@cpoile It's the flaky one again, will merge.