refactor: Service Wiring / Concurrency pt 1

[dave-gray101]

This PR is the start of transitioning our core services to the core/application.go \ core/startup model. In order to keep things reviewable, this PR will have a limited scope - rather than moving everything at once, my goal is to select services that are covered by existing tests for the detection of regressions. Therefore, this PR contains the following components:  

• Finishes up service wiring infrastructure in core/startup
• pkg/concurrency - basic JobResult structure
• Port over TTS Endpoints to TextToSpeechBackendService
• Port over Embedding Endpoint to EmbeddingBackendService

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