YAML Configuration: Replace string values with entity IDs

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Expected Behavior

Configuration entities can be linked together as code (for example, with YAML anchors)

Actual Behavior

Some entities (notably, HTTP connector) require string literals to find linked entities

Corrective Action

Replace string values with entity IDs

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See rev. e6d0a23142e92e14df5f4bc0b5885aece6d8b905