feat: optional external audio mix presets

[Lunkers]

Description

Currently, audio mix presets are tied to the central encore configuration. This is fine in cases where configuration is decoupled from deployments; however, many organizations unfortunately don't set up external configuration servers. In these cases, users cannot add or modify audio mix presets without requiring a new deployment of the service..

This PR adds the option of specifying audio-mix-preset-location under encore-properties.encoding. This allows users to decouple mix presets from the central config, the same way encore currently handles transcoding profiles.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Integration and service-level tests.

Checklist:

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• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)