Add EventFlow integration

Define implementations of the instrumentation abstracts that act as a custom input for https://github.com/Azure/diagnostics-eventflow

This will automatically enable us to support all the telemetry targets supported EventFlow. It will also enable us to use its filtering functionality, to configure exactly what gets reported where.

Start by inspecting the repository's instrumentation abstracts and the Azure diagnostics-eventflow custom input documentation. Define the integration boundary and verify that EventFlow telemetry targets and filtering can be configured through it; the issue names no files or tests to run.

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