Refactor ConferenceService as common util, support custom-hold-music with conference feature

[dremin]

Summary

• ConferenceService actually consisted of all common functions, and other features had to either duplicate it or directly reference it. These services have been combined into a common ProgrammableVoiceService just like the other common serverless classes.
• Enhanced the new ProgrammableVoiceService to accept a custom holdUrl to be set by features
• Updated the custom-hold-music feature to set the ProgrammableVoiceService holdUrl so now when that feature is enabled, other features which use the hold-participant serverless functions also get the custom hold experience

Checklist

• [x] Tested changes end to end
• [x] Requested one or more reviewers

[github-actions[bot]]

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